Method of continuous casting of steel

Abstract

A subject for the invention is to provide a method of continuous steel casting in which although an “immersion nozzle constituted of a refractory material comprising alumina as the main component” is used for continuous steel casting, this nozzle suffers little fusion loss and no alumina deposition thereon and a clean steel casting can be stably obtained.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method of continuous steel casting. More particularly, the invention relates to a method of continuous steel casting characterized by using a combination of a specific mold powder and an immersion nozzle constituted of a refractory material comprising alumina as the main component (e.g., an alumina refractory and/or an alumina-carbon refractory).

BACKGROUND ART

[0002] In continuous steel casting, an immersion nozzle employing an alumina/graphite material containing and/or not containing fused silica as a main-body material and further employing a zirconia/graphite material and/or zirconia/calcia/graphite material as a powder line material is generally used in combination with a mold powder containing a fluorine ingredient.

[0003] The technique using a combination of the immersion nozzle materials and the mold powder material (hereinafter referred to as “related-art technique 1”) is accompanied by the inclusion into the steel of impurities attributable to the refractories and/or attributable to the mold powder material. For avoiding this inclusion, the following techniques have been disclosed.

[0004] With respect to immersion nozzles, a technique for preventing carbon pickup or mold powder suction is known which comprises blowing an inert gas into the molten steel through an immersion nozzle to prevent the molten steel from coming into contact with the nozzle (see JP-A-8-57613 and JP-A-62-130754) (hereinafter referred to as “related-art technique 2-1”).

[0005] Furthermore, an immersion nozzle is known in which a refractory material comprising spinel or a refractory material comprising spinel and periclase is disposed in the part which comes into contact with a molten steel, e.g., low-carbon Al-killed steel, high-oxygen steel, high-Mn steel, stainless steel, or Ca-treated steel, so as to combine unsusceptibility to fusion loss and unsusceptibility to clogging and thereby diminish inclusions attributable to refractories (see JP-A-10-305355) (hereinafter referred to as “related-art technique 2-2”).

[0006] With respect to mold powders, on the other hand, “fluorine ingredient-containing materials”, such as fluorite, capable of forming cuspidine (3CaO.2SiO2.CaF2) crystals are generally used as ones serving as a flux for flowability enhancement and/or contributing to heat abstraction control (hereinafter referred to as “related-art technique 3”).

[0007] However, fluorine ingredients accelerate the fusion loss of the immersion nozzle and indirectly make it difficult to produce clean steel castings. It is hence necessary to use mold powders free of fluorine or having a minimized fluorine ingredient content.

[0008] Known techniques concerning the mold powders free of fluorine, among those mold powders, include:

[0009] a technique in which the pH's of spray cooling water for cooling the casting, secondary cooling water for use after the cooling, and machine-cooling water are kept at a neutral value for the purpose of prolonging the lives of metallic structures, including the pipings in the casting machine main body, and of the concrete facilities (JP-A-58-125349);

[0010] technique in which the pH's of spray cooling water for cooling the casting, secondary cooling water for use after the cooling, and machine-cooling water are likewise kept at a neutral value for the purposes of preventing the casting machine main body, pipings, etc. from corroding and of maintaining flowability and conversion into slag (JP-A-51-93728);

[0011] technique for preventing the generation of fluorine, which is harmful to men and beasts (JP-A-50-86423);

[0012] a technique for preventing environmental pollution, preventing the corrosion of facilities disposed around the continuous-casting machine, and preventing immersion nozzle damage (JP-A-5-208250); and

[0013] a technique for preventing the silicon tetrafluoride formed by reaction with a silicate from impairing the working atmosphere or contaminating the secondary cooling water (JP-A-51-67227). (Hereinafter, these are referred to as “related-art technique 3-1”.)

[0014] Known techniques concerning the mold powders having a minimized fluorine ingredient content include:

[0015] a technique for preventing immersion nozzle damage (JP-A-5-269560); and

[0016] a technique for preventing environmental pollution (JP-A-51-132113). (Hereinafter, these are referred to as “related-art technique 3-2”.)

[0017] Incidentally, in the case of casting with an existing immersion nozzle (see the related-art technique 1), the inner tube and powder line part of the immersion nozzle suffer a fusion loss due to the molten steel, inclusions in the molten steel, mold powder, and slag. This fusion loss changes the shape of the immersion nozzle and disturbs the flow of the molten steel within the mold, resulting in defects in the casting.

[0018] In addition to this “shape change of the immersion nozzle”, a change in the thermal conductivity of the immersion nozzle occurs during casting due to the low-melting and high-melting compounds formed by the reaction of immersion nozzle materials with elements dissolved in the molten steel and/or with the mold powder and slag. Due to this change in thermal conductivity, the quantity of heat abstracted from the molten steel through the immersion nozzle cannot be kept constant. This has resulted in uneven formation of a solidified shell and caused defects in the casting.

[0019] Attempts have been made to overcome those problems by means of a mold powder. Specifically, as stated above, “a mold powder which forms crystals of cuspidine (3CaO.2SiO2.CaF2) as a fluorinated mineral” has been used for controlling the quantity of heat abstracted (see the related-art technique 3). However, the fusion loss in the powder line part is increased, rater than reduced, due to the fluorine ingredient in the mold powder and a sufficient effect has not been obtained so far.

[0020] Application of a mold powder containing no fluorine ingredient or having a low fluorine ingredient content was attempted for the purpose of diminishing the fusion loss in the powder line part (see the related-art techniques 3-1 and 3-2). However, use of this mold powder has made heat abstraction control impossible and caused defects in the casting. Thus, there has been no perfect measure for resolution.

[0021] Although the various measures in continuous steel casting described above have been taken for producing a clean steel casting, these measures also have failed to produce a sufficient effect as will be described below.

[0022] In the related-art technique 2-1, which is “a technique comprising blowing an inert gas into the molten steel through a nozzle to prevent the molten steel from coming into contact with the nozzle”, it is necessary to highly precisely control the rate of inert-gas blowing, blowing angle, size of bubbles, etc. In case where these factors are not controlled, the molten-steel flow is deflected and collides against part of the nozzle, rather than being prevented from contacting the nozzle, leading to a local fusion loss or alumina deposition.

[0023] Furthermore, the mold powder and slag which have been sucked, due to fluctuations in melt surface level caused by the bubbling, into the molten steel filling the mold are caught by the inert gas blown into the molten steel. However, in case where the inert-gas flow is not in a properly controlled state, the nozzle suffers a considerable fusion loss, rather than being prevented from suffering the loss. In this case, since the powder line part is not always in contact with the mold powder, the ordinary nozzle material parts including the nozzle orifice part and inner tube part suffer a fusion loss.

[0024] Once the nozzle suffers a fusion loss, the flow of the inert gas blown through the nozzle is deflected further and this accelerates the fusion loss of the nozzle and/or alumina deposition. The fusion loss of the nozzle results in steel contamination.

[0025] The nozzle according to the related-art technique 2-2, which is “an immersion nozzle in which a refractory material comprising spinel or a refractory material comprising spinel and periclase is disposed in the part which comes into contact with a molten steel”, shows better unsusceptibility to fusion loss in molten steels than alumina/graphite nozzles in ordinary use. A detailed explanation will be given below in this respect.

[0026] The present inventors revealed that the alumina/graphite materials in ordinary use as immersion nozzle materials generally undergo the following reactions with molten steels and are hence undesirable materials for use in producing clean steel castings. Namely, since the molten steel has an exceedingly low carbon concentration, the graphite (C(s) : solid graphite) in the alumina/graphite nozzle material rapidly dissolves in the molten steel through the following reaction.

C(s)→C  Scheme (1)

[0027] Furthermore, (FeO) penetrates into the alumina (Al2O3) in the alumina/graphite material through the following reaction.

Fe(1)+O→(FeO)  Scheme (2)

[0028] Elements dissolved in the molten steel likewise penetrate. For example, in the case of Mn as one of the dissolved elements, (MnO) penetrates into the alumina through the following reaction.

Mn+O→(MnO)  Scheme (3)

[0029] (In schemes (2) and (3), O and Mn represent oxygen and manganese dissolved in the molten steel, and Fe(1) represents an iron ingredient in the molten steel).

[0030] The “Al2O3—FeO” and “Al2O3—MnO” yielded as a result of the penetration of those substances react with inclusions in the molten steel, such as, e.g., “FeO—MnO”, to yield a liquid slag comprising “A12O3—FeO—MnO”. Namely, the alumina suffers a fusion loss due to a combination of the two factors.

[0031] For enhancing spalling resistance, a technique is being generally employed which comprises incorporating fused silica into an alumina/graphite nozzle material. However, this technique is undesirable because fused silica also suffers a fusion loss in a degree equal to or higher than that for alumina.

[0032] In the case of spinel, on the other hand, the amount of (FeO), (MnO), or the like penetrating thereinto is small and, even when inclusions such as FeO—MnO deposit thereon, the spinel retains its solid phase without forming a liquid phase. Namely, the nozzle in which spinel is disposed in the part coming into contact with a molten steel suffers a reduced fusion loss and, hence, brings about a diminution in molten-steel contamination.

[0033] However, production in which the part coming into contact with a molten steel, the main body part, and the powder line part are constituted by disposing different materials leads to an increase in production cost. Furthermore, the use of a spinel material is ineffective in mitigating the fusion loss of the immersion nozzle caused by a powder slag. This is attributable to the fluorine ingredient in the powder slag.

[0034] A technique which is thought to be effective for eliminating those problems is to use a mold powder free of any fluorine ingredient or having a low fluorine ingredient content (see the documents shown above with regard to the prior-art techniques 3-1 and 3-2, i.e., JP-A-58-125349, JP-A-51-93728, JP-A-50-86423, JP-A-5-208250, JP-A-51-67227, JP-A-5-269560, and JP-A-51-132113).

[0035] However, since these mold powders contain no fluorine ingredient or are mold powders having a low fluorine ingredient content, they have poor suitability for viscosity regulation and crystallization temperature regulation and often arouse troubles such as steel breakout and casting cracking, making stable casting impossible. Those mold powders have not been put to practical use so far.

[0036] It can be seen from the above that it is difficult to obtain a clean steel unless the fusion loss of the powder line material of an immersion nozzle is eliminated.

[0037] In view of the above-described problems of prior-art techniques, the present inventors proposed “a specific mold powder (mold powder having a fluorine content lower than 3% by weight and a viscosity at 1,300° C. of from 4 P to 100,000 P)” and developed an invention which is “a method of continuous steel casting comprising using the specific mold powder in combination with a specific immersion nozzle (immersion nozzle comprising: spinel and/or spinel/carbon which constitutes part or all of that part of the immersion nozzle which comes into contact with a molten steel a powder line material constituting the part which comes into contact with the mold powder and/or a slag; and a main body material constituting the other part)” (see JP-A-2001-113345).

[0038] The present inventors made further intensive investigations after the development of that invention. As a result, they have surprisingly found that even when “an immersion nozzle which is constituted of a refractory material comprising alumina as the main component” and in which the powder line part also is constituted of the refractory material is used, then use of the specific mold powder described above enables the nozzle to suffer little fusion loss and no alumina deposition and makes it possible to stably produce a clean steel casting without the need of using this mold powder in combination with the specific immersion nozzle described above. The invention has thus been completed.

[0039] Accordingly, an object of the invention is to provide a method of continuous steel casting which prevents steel contamination due to refractories and makes it possible to stably produce a highly clean steel casting and which further has an effect from the standpoint of immersion nozzle production that the nozzle can be produced exceedingly easily because the same “refractory material comprising alumina as the main component” is used.

[0040] Disclosure of the Invention

[0041] The present inventors made intensive investigations in order to overcome the problems described above and to accomplish the object. As a result, based on that finding, the inventors have invented “a method of continuous steel casting which comprises continuously casting a steel while feeding a molten steel into a casting mold through an immersion nozzle and supplying a mold powder into the casting mold, characterized by using a combination of a mold powder having a fluorine content lower than 3% by weight and a viscosity at 1,300° C. of from 4 P to 100,000 P and an immersion nozzle constituted of a refractory material comprising alumina as the main component”.

[0042] It has hitherto been essential to use a “fluorine ingredient” for reducing the viscosity of a mold powder and for heat abstraction control. However, the inventors have found that when a slag film having evenness in properties and/or thickness is formed between the mold and a solidified shell, then there is no need of relying on a fluorine ingredient. Namely, it has been found that increasing the viscosity of a mold powder enables the formation of an even slag film and this slag film performs the function of cuspidine (3CaO.2SiO2.CaF2) (heat abstraction control).

[0043] It has further been found that when the mold powder has a rupture strength at 1,300° C. of 3.7 g/cm2 or higher, a continuous slag film can be formed and continuous casting is possible. The mold powder to be used in the invention preferably is one having a chemical composition comprising from 5 to 25% by weight Al2O3, from 25 to 70% by weight SiO2, from 10 to 50% by weight CaO, up to 20% by weight MgO, and from 0 to 2% by weight F (unavoidable impurity).

[0044] The immersion nozzle to be used in combination with the mold powder is an immersion nozzle constituted of a refractory material comprising alumina as the main component. For example, the refractory material comprises an alumina refractory and/or an alumina-carbon refractory. It has been further found that the immersion nozzle can be one in which the refractory “contains one or more members selected from silica (SiO2) , silicon carbide (SiC), boron carbide (B4C), silicon nitride (Si3N4), aluminum nitride (AlN), zirconium boride (ZrB2), magnesium boride (Mg3B2), zirconium sulfate (ZrSO4), silicon (Si), and aluminum (Al)”.

[0045] The molten steel to be cast may be any of all kinds of steels, such as, e.g., aluminum-killed steel, silicon-killed steel, high-oxygen steel, stainless steel, steel for electromagnetic steel sheets, calcium-treated steel, high-manganese steel, free-cutting steel, boron steel, steel cord, case hardening steel, or high-titanium steel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] FIG. 1 is a view illustrating an example of the structure of the immersion nozzle of the type having orifice parts which is to be used in Examples according to the invention (and in Comparative Examples).

[0047] FIG. 2 is a view illustrating another example of the structure of the immersion nozzle of the type having orifice parts which is to be used in Examples according to the invention (and in Comparative Examples).

[0048] FIG. 3 is a view illustrating an example of the straight type immersion nozzle having no orifice part which is to be used in Examples according to the invention (and in Comparative Examples).

[0049] In the figures, numeral 1 denotes an immersion-nozzle inner tube part, which comes into contact with a molten steel, 2 an immersion-nozzle orifice part, which comes into contact with a molten steel, 3 an immersion-nozzle powder line part, which comes into contact with a mold powder, 4 an immersion-nozzle main body part, and 5 a straight type immersion-nozzle tip part, which comes into contact with a molten steel.

BEST MODE FOR CARRYING OUT THE INVENTION

[0050] Modes for carrying out the invention will be explained below. The mold powder to be used in the invention is one having a fluorine content lower than 3% by weight and a viscosity at 1,300° C. of from 4 to 100,000 P, as stated above.

[0051] In case where the fluorine content in the mold powder is 3% by weight or higher, the immersion nozzle suffers an increased fusion loss especially in the powder line part and the refractory ingredients which have come into the steel contaminate the molten steel, making it impossible to obtain a clean steel.

[0052] Viscosities of the mold powder (viscosities at 1,300° C.) lower than 4 P are undesirable because an uneven mold powder flow generates and crystals of dicalcium silicate, tricalcium silicate, and the like grow in the molten mold powder, resulting in increased temperature fluctuations of the mold copper plates and in unstable heat abstraction. On the other hand, viscosities thereof exceeding 100,000 are undesirable because the powder shows poor fusibility and a slag bear generates, making stable casting impossible.

[0053] In the invention, the viscosity can be regulated, for example, with Al2O3, CaO/SiO2, or the like. When the Al2O3 content is high or the CaO/SiO2 content is low, the viscosity can be regulated so as to be high.

[0054] The mold powder to be used in the invention preferably further has a rupture strength at 1,300° C. of 3.7 g/cm2 or higher, provided that the “rupture strength of the melted mold powder” is defined as the maximum load as measure at the time when a cylindrical platinum rod with a diameter of 7 mm which is being pulled out of the melt at a constant rate separates from the liquid surface and the liquefied mold powder breaks into droplets. Rupture strengths lower than 3.7 g/cm2 are undesirable because the liquid layer in a slag film is apt to break.

[0055] The mold powder to be used in the invention can be produced from a base raw material such as portland cement, wollastonite, or synthetic calcium silicate, an SiO2 source such as perlite or fly ash, an Na2O, K2O, or Li2O source such as a carbonate, glass powder, or frit powder, an MgO source such as magnesium carbonate, MgO powder from seawater, or dolomite powder, a B2O3 source such as borax, colemanite, glass powder, or frit powder, and a carbonaceous raw material such as coke powder, flaky graphite, or carbon black. However, fluorides such as NaF and CaF2 are not included.

[0056] Specifically, the mold powder can be produced by suitably adding the SiO2, Na2O, K2O, Li2O, MgO, and B2O3 sources and the carbonaceous raw material to the basic raw material and regulating the viscosity with Al2O3, CaO/SiO2, or the like as stated above. For example, the raw materials are mixed together in such a proportion as to result in a chemical composition which comprises from 5 to 25% by weight Al2O3, from 25 to 70% by weight SiO2, from 10 to 50% by weight CaO, from 3 to 20% by weight one or more members selected from the group consisting of Na2O, Li2O, and K2O, up to 20% by weight MgO, up to 3% by weight fluorine ingredient as an unavoidable impurity, and from 0.5 to 8% by weight carbon and in which the CaO/SiO2 weight ratio is in the range of from 0.2 to 1.5. Thereafter, this mixture is homogenized with a mixer to thereby obtain the mold powder.

[0057] It is also possible to use the mold powder in a granular form prepared by adding a liquid (e.g., water) to the powder optionally together with an organic binder or inorganic binder and granulating the mixture by a technique such as extrusion granulation, stirring granulation, rolling granulation, flow granulation, or spray granulation.

[0058] Material embodiments of the immersion nozzle to be used in combination with the mold powder described above will be explained next.

[0059] The material constituting the immersion nozzle in the invention is a refractory material comprising alumina as the main component. A preferred embodiment thereof is an alumina refractory and/or an alumina-carbon refractory.

[0060] The alumina refractory and alumina-carbon refractory may be ones which contain one or more members selected from silica (SiO2), silicon carbide (SiC), boron carbide (B4C), silicon nitride (Si3N4), aluminum nitride (AlN), zirconium boride (ZrB2), magnesium boride (Mg3B2), and zirconium sulfate (ZrSO4). Such a wide range of materials can be used.

[0061] In another preferred embodiment, the refractories contain one or more of silicon (Si) and aluminum (Al). As a result of the incorporation of such a metal, the metal reacts with the refractory material in especially the powder line part of the immersion nozzle and/or with a component of the air during use at high temperatures to yield a metal reaction product. This metal reaction product strengthens the powder line part and contributes to an improvement in life. In the case where the powder line part contains carbon, the metal functions also as an antioxidant for the carbon. Thus, by incorporating the metal, an excellent immersion nozzle can be provided. The content of silicon (Si) and aluminum (Al) is preferably from 0.1 to 15% by weight, more preferably from 1 to 8% by weight. Contents thereof lower than 0.1% by weight are undesirable because those effects of the metal cannot be obtained. Contents thereof exceeding 15% by weight are undesirable because a metal reaction product is yielded in a large amount and this leads to destruction of the refractory structure by the resultant volume increase and to the loss of effects of the main component of the refractory material.

[0062] In the invention, the powder line part and main body part of the immersion nozzle can be made of the same material. The reason for this is that the specific mold powder according to the invention (mold powder having a fluorine content lower than 3% by weight and a viscosity at 1,300° C. of from 4 P to 100,000 P) is used.

[0063] Hitherto, a zirconia/carbon material has been mainly used as a material having high unsusceptibility to fusion loss by mold powders containing a fluorine ingredient. Compared to general refractory materials, this material is costly. In addition, even with this material, the powder line part suffers a considerable fusion loss and there have been cases where this fusion loss is a factor determining the life of the immersion nozzle.

[0064] However, according to the invention, use of the high-viscosity mold powder containing almost no fluorine ingredient or containing no fluorine ingredient has eliminated the fusion loss by a fluorine ingredient almost completely or completely. Because of this, there is no need of using a zirconia/carbon material for constituting the powder line part and the material described above (refractory material comprising alumina as the main component) can be freely used for the powder line part. As a result, it has become possible to use the same material as that constituting the main body part.

EXAMPLES

[0065] The invention will be explained below in detail by reference to Examples and Comparative Examples, but the invention should not be construed as being limited to the following Examples.

[0066] The structures of immersion nozzles usable in the following Examples and Comparative Examples will be explained below by reference to FIGS. 1 to 3. FIG. 1 is a view illustrating an example of immersion nozzle structures of the type having orifice parts, and FIG. 2 is a view illustrating another example of immersion nozzle structures also having orifice parts. FIG. 3 is a view illustrating an example of straight type immersion nozzles having no orifice part.

[0067] The immersion nozzle shown in FIG. 1 is an immersion nozzle of the type having orifice parts. In FIG. 1, 1 denotes an immersion-nozzle inner tube part, which comes into contact with a molten steel; 2 denotes an immersion-nozzle orifice part, which also comes into contact with a molten steel; 3 denotes a powder line part, which comes into contact with a mold powder and/or a slag; and 4 denotes a main part of the immersion nozzle.

[0068] As shown in FIG. 1, this immersion nozzle has a structure in which the orifice parts 2 have been united with the main body part 4 to constitute that region 2a of the immersion nozzle orifice part which comes into contact with a molten steel.

[0069] The immersion nozzle shown in FIG. 2 is an immersion nozzle of the type having orifice parts like the immersion nozzle shown in FIG. 1. However, this nozzle does not have the united structure such as that shown in FIG. 1 (see “region 2a” in FIG. 1), but is an immersion nozzle having a structure in which that region 2b of the immersion-nozzle orifice parts 2 which comes into contact with a molten steel is made of the same material. In FIG. 2, numerates 1 to 4 have the same meanings as shown above, i.e., 1 denotes an inner tube part, 2 an orifice part, 3 a powder line part, and 4 a main body part.

[0070] The immersion nozzle shown in FIG. 3 is a straight type immersion nozzle having no orifice part unlike the immersion nozzles shown in FIGS. 1 and 2. In FIG. 3, numeral 5 denotes a nozzle tip part, which comes into contact with a molten steel, and the other numerals have the same meanings as shown above, i.e., 1 denotes an inner tube part, 3 a powder line part, and 4 a main body part.

[0071] The chemical compositions of the mold powders (sample Nos. 1 to 7) used in the following Examples are shown in Table 1, and the chemical compositions of comparative mold powders (sample Nos. 8 to 21) are shown in Table 2. In Tables 1 and 2 are further shown the “fluorine ingredient”, “viscosity (at 1,300° C.)”, and “rupture strength (at 1,300° C.)” of each mold powder.

[0072] Incidentally, the mold powders of sample Nos. 1 to 7, 8 to 10, and 13 to 17 shown in Tables 1 and 2 are “powdered products” obtained by mixing by means of a mixer so as to result in the given chemical compositions. The other mold powders, i.e., sample Nos. 11, 12, and 18 to 21, were “granulated products” obtained by mixing raw powders, subsequently adding a solution consisting of 90% by weight water and 10% by weight sodium silicate thereto in an amount of from 20 to 30% by weight to produce a slurry, and spray-granulating and drying the slurry. These granulated products have been regulated so as to finally have the given chemical compositions. 1 TABLE 1 Chemical compositions of mold powders used in Examples Sample No. 1 2 3 4 5 6 7 Chemical SiO2 36 39 50 49 48 31 31 composi- Al2O3 7 21 10 10 18 7 7 tion of CaO 36 35 20 19 16 43 43 mold MgO 4 1 10 10 8 6 8 powder, Na2O + Li2O + K2O 5 2 6 8 6 8 6 wt % MnO + BaO + SrO + B2O3 8 0 1 1 1 0 0 F 1 0 0 0 0 2 2 Total carbon amount 3 2 3 3 3 3 3 CaO/SiO2 1.00 0.90 0.40 0.39 0.33 1.40 1.40 (weight ratio) Fluorine ingredient (wt %) 1 0 0 0 0 2 2 Viscosity (at 1300° C.) (P) 30 20 40 50 100 5 5 Rupture strength (at 1300° C.) 5 8 10 3.7 5 6 5 (g/cm2)

[0073] 2 TABLE 2(1) Chemical compositions of comparative mold powders Sample No. 8 9 10 11 12 13 14 Chemical SiO2 25 26 34 32 27 29 29 composi- Al2O3 9 5 3 3 2 12 11 tion of CaO 28 27 38 37 32 30 32 mold MgO 7 9 10 12 10 11 7 powder, Na2O + Li2O + K2O 21 20 3 3 10 6 5 wt % MnO + BaO + SrO + B2O3 0 0 0 0 1 1 4 F 5 8 8 10 12 8 9 Total carbon amount 5 5 4 3 6 3 3 CaO/SiO2 1.12 1.04 1.12 1.16 1.19 1.05 1.10 (weight ratio) Fluorine ingredient (wt %) 5 8 8 10 12 8 9 Viscosity (at 1300° C.) (P) 2.0 1.5 1.5 0.5 1.2 2.0 1.0 Rupture strength (at 1300° C.) 3.5 3.0 3.2 1.0 2.5 3.0 2.0 (g/cm2)

[0074] 3 TABLE 2(2) Chemical compositions of comparative mold powders Sample No. 15 16 17 18 19 20 21 Chemical SiO2 30 30 29 27 26 25 22 composi- Al2O3 10 7 5 9 8 7 4 tion of CaO 34 36 38 40 42 42 38 mold MgO 6 8 3 5 5 0 8 powder, Na2O + Li2O + K2O 8 6 10 3 2 5 6 wt % MnO + BaO + SrO + B2O3 0 0 0 0 0 0 0 F 9 10 11 13 15 18 19 Total carbon amount 3 3 4 3 2 3 3 CaO/SiO2 1.12 1.20 1.30 1.50 1.60 1.65 1.70 (weight ratio) Fluorine ingredient (wt %) 9 10 11 13 15 18 19 Viscosity (at 1300° C.) (P) 1.8 1.3 1.0 0.9 0.8 0.3 0.2 Rupture strength (at 1300° C.) 2.7 2.4 2.8 1.3 1.5 0.7 0.5 (g/cm2)

Examples 1 to 17 and Comparative Examples 1 to 6

[0075] Examples 1 to 17 are shown in Tables 3 and 4, and Comparative Examples 1 to 6 are shown in Table 5.

[0076] In each of the following Examples 1 to 17 and Comparative Examples 1 to 6, continuous casting was conducted while feeding a molten steel (“Kind of steel” in the tables) into a casting mold through a nozzle and simultaneously supplying a mold powder into the casting mold. The structure of the nozzle used in each Example or Comparative Example is shown in the table in terms of figure number. The mold powders used in the Examples and Comparative Examples had the chemical compositions of sample Nos. 1 to 21 shown in Tables 1 and 2, and the respective sample Nos. are shown in Tables 3 to 5. Only the “fluorine ingredient”, “viscosity (at 1,300° C.)”, and “rupture strength (at 1,300° C.)” of each mold powder used are shown therein. In the tables, “%” for the material in each nozzle part in the tables means “% by weight”.

[0077] In each of the Examples and Comparative Examples, “stable casting”, “nozzle fusion loss or alumina deposit amount (fusion loss of each of inner tube, orifice part inside, and powder line)”, “steel cleanness”, and “percentage of steel defects” were evaluated in the following manners, and the results of the evaluations are shown in Tables 3 to 5.

[0078] Evaluation of Stable Casting

[0079] “Stable casting” indicates whether stable casting is possible or not. The case in which no BO warning [method of evaluation employing a system which foresees the occurrence of B.O (breakout) based on a continuous measurement of the mold surface temperature] was given during casting and in which the immersion nozzle had no fusion rupture accident [accident in which the immersion nozzle breaks during casting due to a fusion loss of the powder line and/or a part in contact with the molten steel] is indicated by “possible”, while the other cases are indicated by “impossible”.

[0080] Evaluation of Nozzle Fusion Loss

[0081] “Nozzle fusion loss [mm/(steel ton)]” is given in terms of the dimension of the nozzle fusion loss per ton of the steel cast. As the nozzle fusion loss increases, not only the nozzle life becomes short, but also the amount of impurities coming into the steel as a result of the fusion loss increases and the steel is more contaminated accordingly.

[0082] Alumina Deposit Amount

[0083] The amount of alumina deposited when aluminum-killed steel was cast is shown. Alumina deposition occurs on the inner tube and/or orifice inside in the nozzle. Large alumina deposit amounts make stable casting impossible. In some cases, the deposit prevents the molten steel from passing through the nozzle, resulting in casting stoppage. Consequently, the less the alumina deposition, the better the nozzle.

[0084] Evaluation of Steel Cleanness

[0085] “Steel cleanness” was evaluated in terms of the degree of sliver mars. Index “100” indicates the case in which the steel has no sliver defects, while index “0” indicates the case in which the steel cannot be a commercial product due to sliver defects. Indexes between these values were statistically graded for the evaluation.

[0086] Evaluation of Percentage of Steel Defects

[0087] “Percentage of steel defects” was evaluated based on surface cracks. The case in which the steel has negligible surface cracks is indicated by “◯”, the case in which the steel cannot be a commercial product due to surface cracks is indicated by “X”, and the case in which the steel can be a commercial product by processing the steel surface is indicated by “&Dgr;”. 4 TABLE 3(1) Examples 1-5 Example 1 2 3 4 5 Structure (FIG. 1) (FIG. 2) (FIG. 1) (FIG. 2) (FIG. 1) Nozzle Noz- Material zle part Inner Al2O3 80 70 90 60 65 tube C 20 30 10 30 30 part SiO2 0 0 0 10 5 Kind of — — — — — additive Additive — — — — — amount Ori- Al2O3 80 70 90 60 65 fice C 20 30 10 30 30 part SiO2 0 0 0 10 5 Kind of — — — — — additive Additive — — — — — amount Pow- Al2O3 80 70 90 60 65 der line C 20 30 10 30 30 part SiO2 0 0 0 10 5 and Kind of — — — — — main additive body Additive — — — — — part amount Mold Sample No. [5] [1] [7] [4] [1] pow- Fluorine 0 1 2 0 1 der ingredient (wt %) Viscosity 100 30 5 50 30 (at 1300° C.) (P) Rupture strength 5.0 5.0 5.0 3.7 5.0 (at 1300° C.) (g/cm2) Kind of steel Al-killed Al-killed Al-killed Al-killed Al-killed Evalu- Stable casting possible possible possible possible possible ation Nozzle Inner 0 0 0 0 0 fusion tube loss or Orifice 0 0 0 0 0 alumi- inside na dep- Powder 0 0 0 0 0 osi- line tion Steel cleanness 100 100 100 100 100 Percentage of ◯ ◯ ◯ ◯ ◯ steel defects

[0088] 5 TABLE 3(2) Examples 6-10 Example 6 7 8 9 10 Structure (FIG. 1) (FIG. 2) (FIG. 1) (FIG. 2) (FIG. 1) Nozzle Noz- Material zle part Inner Al2O3 70 45 70 60 100 tube C 30 35 30 25 0 part SiO2 0 20 0 15 0 Kind of SiC:B4C Si:SiC Si3N4:Al AlN:ZrB2 Mg3B2:Al additive Additive 3:3 2:5 3:2 5:4 2:3 amount Ori- Al2O3 70 45 70 60 100 fice C 30 35 30 25 0 part SiO2 0 20 0 15 0 Kind of SiC:B4C Si:SiC Si3N4:Al AlN:ZrB2 Mg3B2:Al additive Additive 3:3 2:5 3:2 5:4 2:3 amount Pow- Al2O3 70 45 70 60 100 der C 30 35 30 25 0 line SiO2 0 20 0 15 0 part Kind of SiC:B4C Si:SiC Si3N4:Al AlN:ZrB2 Mg3B2:Al and additive main Additive 3:3 2:5 3:2 5:4 2:3 body amount part Mold Sample No. [6] [3] [1] [6] [2] pow- Fluorine 2 0 1 2 0 der ingredient (wt %) Viscosity 5 40 30 5 20 (at 1300° C.) (P) Rupture strength 6.0 10 5.0 6.0 8.0 (at 1300° C.) (g/cm2) Kind of steel high- stainless Si-killed electro- Ca- oxygen steel steel magnetic treated steel steel steel sheet Evalu- Stable casting possible possible possible possible possible ation Nozzle Inner 0 0 0 0 0 fusion tube loss or Orifice 0 0 0 0 0 alumi- inside na dep- Powder 0 0 0 0 0 osi- line tion Steel cleanness 100 100 100 100 100 Percentage of ◯ ◯ ◯ ◯ ◯ steel defects

[0089] 6 TABLE 4 Examples 11-17 Example 11 12 13 14 15 16 17 Structure (FIG. 2) (FIG. 2) (FIG. 2) (FIG. 2) (FIG. 2) (FIG. 2) (FIG. 2) Nozzle Noz- Material zle part Inner Al2O3 65 100 100 100 100 100 100 tube C 30 — — — — — — part SiO2 5 — — — — — — Kind of ZrSiO4 — ZrO2 — — — — additive Additive 5 — 20 — — — — amount Ori- Al2O3 65 100 50 70 75 80 90 fice C 30 — 30 30 25 20 10 part SiO2 5 — — — — — — Kind of ZrSiO4 — ZrO2 — — — — additive Additive 5 — 20 — — — — amount Pow- Al2O3 65 100 100 70 75 80 90 der C 30 — — 30 25 20 10 line SiO2 5 — — — — — — part Kind of ZrSiO4 — — — — — — and additive main Additive 5 — — — — — — body amount part Mold Sample No. [1] [7] [4] [4] [3] [5] [2] pow- Fluorine 1 2 0 0 0 0 0 der ingredient (wt %) Viscosity 30 5 50 50 40 100 20 (at 1300° C.) (P) Rupture strength 5.0 5 3.7 10 5 8 1.0 (at 1300° C.) (g/cm2) Kind of steel high- case Al- Ca- elec- stain- steel Mn hard- killed treat- tro- less cord steel ening ed mag- steel steel steel netic steel sheet Evalu- Stable casting possi- possi- possi- possi- possi- possi- possi- ation ble ble ble ble ble ble ble Nozzle Inner 0 0 0 0 0 0 0 fusion tube loss or Orifice 0 0 0 0 0 0 0 alumi- inside na dep- Powder 0 0 0 0 0 0 0 osi- line tion Steel cleanness 100 100 100 100 100 100 100 Percentage of ◯ ◯ ◯ ◯ ◯ ◯ ◯ steel defects

[0090] 7 TABLE 5 Comparative Examples 1-6 Comparative Example 1 2 3 4 5 6 Structure (FIG. 1) (FIG. 2) (FIG. 2) (FIG. 2) (FIG. 2) (FIG. 1) Noz- Noz- Material zle zle part Inner Al2O3 100 100 100 70 80 100 tube C — — — 30 20 — part SiO2 — — — — — — Kind of — — — — — — additive Additive — — — — — — amount Ori- Al2O3 100 70 90 70 80 100 fice C — 30 10 30 20 — part SiO2 — — — — — — Kind of — — — — — — additive Additive — — — — — — amount Pow- Al2O3 100 70 90 60 70 50 der C — 30 10 20 20 20 line SiO2 — — — — 10 — part Kind of — — — ZrO2 — ZrO2 and additive main Additive — — — 20 — 30 body amount part Mold Sample No. [8] [10] [12] [20] [13] [11] pow- Fluorine 5 8 12 18 8 10 der ingredient (wt %) Viscosity 2 1.5 1.2 0.3 2.0 0.5 (at 1300° C.) (P) Rupture strength 3.5 3.2 2.5 0.7 3.0 1.0 (at 1300° C.) (g/cm2) Kind of steel elec- Ca- steel Al- stain- high- tro- treated cord killed less oxygen mag- steel steel steel netic steel sheet Evalu- Stable casting impossi- impossi- impossi- impossi- impossi- impossi- ation ble ble ble ble ble ble Nozzle Inner 0.02 0.03 0.04 0.01 0.05 0.02 fusion tube loss or Orifice 0.07 0.05 0.08 0.04 0.09 0.03 alumi- inside na dep- Powder 0.50 0.30 0.60 0.20 0.60 0.40 osi- line tion Steel cleanness 30 40 30 50 20 40 Percentage of X X X X X X steel defects

[0091] Tables 3 to 5 show the following. When the mold powders specified in the invention were used, “evaluation of stable casting” was “possible”, i.e., stable casting was possible, even with each of the immersion nozzles in which the inner tube part, orifice parts, powder line part, and main body part were all made of an alumina/carbon refractory (Examples 1 to 11) and the immersion nozzle in which these parts were all made of an alumina refractory (Example 12), i.e., even with the same refractory. Moreover, even when the powder line part and the main body part were made of the same material as in Examples 1 to 17, stable casting was likewise possible.

[0092] Furthermore, the “nozzle fusion loss or alumina deposition” was “0” in each case and the “steel cleanness” was “100”. The “percentage of steel defects” also was “◯” in each case and the surface cracks in each steel were negligible.

[0093] In contrast, in each of Comparative Examples 1 to 6, in which the mold powder specified in the invention was not used, “stable casting” was “impossible”, i.e., the steel could not be stably cast, as apparent from Table 5. The Comparative Examples were inferior also in “nozzle fusion loss”, “steel cleanness”, and “percentage of steel defects”. Furthermore, even the nozzles in which the inner tube part was made of the same material as in Examples were inferior. This is because during casting, a flow in a direction opposite to the direction of the flow of the molten steel occurs simultaneously. Because of this, the powder is carried by the reverse flow and comes into contact with the inner tube part to cause a fusion loss, etc., giving the poor results shown in Table 5.

[0094] A comparison between the evaluation results for Comparative Examples 1 to 6 and the evaluation results for Examples 1 to 17 according to the invention shows the following. Stable casting was possible only when the mold powders specified in the invention were used. Since the nozzles suffered an extremely reduced fusion loss, the nozzle life was improved. Furthermore, almost no sliver defects were observed and the steels had negligible surface cracks.

[0095] The immersion nozzles of FIGS. 1 and 2 used in Examples 1 to 17 given above (the immersion nozzle of FIG. 3 also is usable of course) and the mold powders shown in Table 1 are mere examples for the invention. The invention is not limited to such constitutions, and various combinations within a range specifying the invention can be used.

[0096] Industrial Applicability

[0097] As described above in detail, the invention is characterized by a method of continuous steel casting in which a mold powder containing virtually no fluorine ingredient, which enhances fusion loss, is used in combination with an immersion nozzle constituted of a refractory material comprising alumina as the main component.

[0098] Due to this constitution, the following marked effects are produced. Impurities attributable to refractory ingredients are prevented from coming into the molten steel and alumina deposition within the nozzle is inhibited. Consequently, stable casting is possible and an ultraclean steel can be obtained. In addition, since defects in the casting which are attributable to a refractory have been considerably diminished, the yield of castings is improved.

[0099] The immersion nozzle to be used in the invention suffers almost no fusion loss and, hence, can have an improved nozzle life. It has high performance and is inexpensive due to a reduced wall thickness and a reduced weight. The use of this immersion nozzle in combination with the mold powder specified in the invention produces an industrially exceedingly highly valuable effect that the combination is applicable to all kinds of steels such as, e.g., aluminum-killed steel, silicon-killed steel, high-oxygen steel, stainless steel, steel for electromagnetic steel sheets, calcium-treated steel, high-manganese steel, free-cutting steel, boron steel, steel cord, case hardening steel, or high-titanium steel.

[0100] Furthermore, from the standpoint of immersion nozzle production, the nozzle has an advantage that it can be produced exceedingly easily because the same “refractory material comprising alumina as the main component” is used.

Claims

1. A method of continuous steel casting which comprises continuously casting a steel while feeding a molten steel into a casting mold through an immersion nozzle and supplying a mold powder into the casting mold,

wherein a combination of a mold powder having a fluorine content lower than 3% by weight and a viscosity at 1,300° C. of from 4 P to 100,000 P and an immersion nozzle comprising a refractory material comprising alumina as a main component is used.

2. The method of continuous steel casting of claim 1, wherein the mold powder has a rupture strength at 1,300° C. of 3.7 g/cm2 or higher.

3. The method of continuous steel casting of claim 1 or 2, wherein the mold powder has a chemical composition comprising from 5 to 25% by weight of Al2O3, from 25 to 70% by weight of SiO2, from 10 to 50% by weight of CaO, up to 20% by weight of MgO, and from 0 to 2% by weight of F (unavoidable impurity).

4. The method of continuous steel casting of claim 1, wherein the refractory material comprises an alumina refractory and/or an alumina-carbon refractory.

5. The method of continuous steel casting of claim 4, wherein the alumina refractory and/or alumina-carbon refractory contains one or more members selected from silica (SiO2), silicon carbide (SiC), boron carbide (B4C), silicon nitride (Si3N4), aluminum nitride (AlN), zirconium boride (ZrB2), magnesium boride (Mg3B2), zirconium sulfate (ZrSO4), silicon (Si), and aluminum (Al).

6. The method of continuous steel casting of any one of claims 1 to 5, wherein as the molten steel, aluminum-killed steel, silicon-killed steel, high-oxygen steel, stainless steel, steel for electromagnetic steel sheets, calcium-treated steel, high-manganese steel, free-cutting steel, boron steel, steel cord, case hardening steel, or high-titanium steel is used.