MOLD POWDER FOR CONTINUOUS CASTING OF Al-CONTAINING SUB-PERITECTIC STEEL AND CONTINUOUS CASTING METHOD

Abstract

A mold powder which prevents surface defects from occurring on a surface of a cast slab of Al-containing hypo-peritectic steel having Al: 0.2% to 2.0%, by mass %, and, in a hypo-peritectic region, C: 0.08% to 0.17%, by mass %. The mold powder includes CaO, SiO.sub.2, Na.sub.2O, Li.sub.2O, F, and C. Li.sub.2O/Na.sub.2O is 0.6 or more, 1.0+0.05×Al≤CaO/SiO.sub.2≤2.0−0.35×Al, 10<Li.sub.2O+0.5×Na.sub.2O+0.8×F<20, and 1.00≤F/(Li.sub.2O+0.5×Na.sub.2O+1.46)≤1.24 are satisfied where Al is content by mass % of molten steel, and respective contents of the remaining elements are by mass %. A viscosity of the mold powder at 1,300° C. is in a range of 0.05 Pa.Math.s to 0.20 Pa.Math.s, and a crystallization temperature of the mold powder is in a range of 1,100° C. to 1,250° C.

Claims

1. A mold powder for continuous casting of Al-containing hypo-peritectic steel, the Al-containing hypo-peritectic steel having Al: 0.2% to 2.0%, and, in a hypo-peritectic region, C: 0.08% to 0.17%, by mass %, the mold powder comprising, by mass %: CaO; SiO.sub.2; Na.sub.2O: 8% or less; Li.sub.2O: 2% to 7%; F; and C: 2% to 10%, wherein Li.sub.2O/Na.sub.2O is 0.6 or more, where respective contents are by mass %, inequalities (1), (2), and (3) are satisfied: $\begin{array}{cc}1.0+0.05\times \mathrm{Al}\le \mathrm{CaO}/{\mathrm{SiO}}_2\le 2.0-0.35\times \mathrm{Al}& \left(1\right)\\ 10<{\mathrm{Li}}_{2}O+0.5\times {\mathrm{Na}}_{2}O+0.8\times F<20& \left(2\right)\\ 1.00\le F/\left({Li}_{2}O+0.5\times {\mathrm{Na}}_{2}O+1.46\right)\le 1.24& \left(3\right)\end{array}$ where Al is content by mass % of molten steel, and respective contents of the remaining elements are by mass %, a viscosity of the mold powder at 1,300° C. is in a range of 0.05 Pa.Math.s to 0.20 Pa.Math.s, and a crystallization temperature of the mold powder is in a range of 1,100° C. to 1,250° C.

2. The mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 1, wherein the mold powder further comprises, by mass %, at least one selected from the group consisting of K.sub.2O: 5% or less, MnO: 5% or less, MgO: 5% or less, B.sub.2O.sub.3: 5% or less, and BaO: 5% or less.

3. The mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 1, wherein the mold powder further comprises, by mass %, Al.sub.2O.sub.3: 3% or less.

4. The mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 1, wherein a viscosity difference (Δη=η.sub.1−η.sub.0) between a viscosity η.sub.1 and a viscosity η.sub.0 is 0.15 Pa.Math.s or less, and a crystallization temperature difference (ΔT.sub.CS=T.sub.CS1−T.sub.CS0) between a crystallization temperature T.sub.CS1 and a crystallization temperature T.sub.CS0 is 100° C. or less, where η.sub.0 is a viscosity at 1,300° C. in an initial composition of the mold powder, T.sub.CS0 is a crystallization temperature in the initial composition, η.sub.1 is a viscosity at 1,300° C. in a composition of the mold powder in which SiO.sub.2 of the mold powder is reduced from the initial composition by 17.6%, by mass % and Al.sub.2O.sub.3 content is increased from the initial composition by 20.0%, by mass %, and T.sub.CS1 is a crystallization temperature in the composition.

5. A method for continuously casting Al-containing hypo-peritectic steel, the method comprising: supplying the mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 1 into a mold for continuous casting when Al-containing hypo-peritectic steel having Al: 0.2% to 2.0%, by mass %, and, in a hypo-peritectic region, C: 0.08% to 0.17%, by mass %, is continuously cast, wherein a slab drawing speed is in a range of 0.7 m/min to 2.0 m/min, and a thickness of a mold powder molten layer is in a range of 8×Q.sup.1/2 mm to 18×Q.sup.1/2 mm for a casting flow rate (Q; tons/min) of molten steel and is 35 mm or less.

6. The mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 1, wherein the mold powder further comprises, by mass %, Al.sub.2O.sub.3: 3% or less.

7. The mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 2, wherein a viscosity difference (Δη=η.sub.1−η.sub.0) between a viscosity η.sub.1 and a viscosity η.sub.0 is 0.15 Pa.Math.s or less, and a crystallization temperature difference (ΔT.sub.CS=T.sub.CS1−T.sub.CS0) between a crystallization temperature T.sub.CS1 and a crystallization temperature T.sub.CS0 is 100° C. or less, where η.sub.0 is a viscosity at 1,300° C. in an initial composition of the mold powder, T.sub.CS0 is a crystallization temperature in the initial composition, η.sub.1 is a viscosity at 1,300° C. in a composition of the mold powder in which SiO.sub.2 of the mold powder is reduced from the initial composition by 17.6%, by mass %, and Al.sub.2O.sub.3 content is increased from the initial composition by 20.0%, by mass %, and T.sub.CS1 is a crystallization temperature in the composition.

8. The mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 3, wherein a viscosity difference (Δη=η.sub.1−η.sub.0) between a viscosity η.sub.1 and a viscosity η.sub.0 is 0.15 Pa.Math.s or less, and a crystallization temperature difference (ΔT.sub.CS=T.sub.CS1−T.sub.CS0) between a crystallization temperature T.sub.CS1 and a crystallization temperature T.sub.CS0 is 100° C. or less, where η.sub.0 is a viscosity at 1,300° C. in an initial composition of the mold powder, T.sub.CS0 is a crystallization temperature in the initial composition, η.sub.1 is a viscosity at 1,300° C. in a composition of the mold powder in which SiO.sub.2 of the mold powder is reduced from the initial composition by 17.6%, by mass %, and Al.sub.2O.sub.3 content is increased from the initial composition by 20.0%, by mass %, and T.sub.CS1 is a crystallization temperature in the composition.

9. The mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 6, wherein a viscosity difference (Δη=η.sub.1−η.sub.0) between a viscosity η.sub.1 and a viscosity η.sub.0 is 0.15 Pa.Math.s or less, and a crystallization temperature difference (ΔT.sub.CS=T.sub.CS1−T.sub.CS0) between a crystallization temperature T.sub.CS1 and a crystallization temperature T.sub.CS0 is 100° C. or less, where η.sub.0 is a viscosity at 1,300° C. in an initial composition of the mold powder, T.sub.CS0 is a crystallization temperature in the initial composition, η.sub.1 is a viscosity at 1,300° C. in a composition of the mold powder in which SiO.sub.2 of the mold powder is reduced from the initial composition by 17.6%, by mass %, and Al.sub.2O.sub.3 content is increased from the initial composition by 20.0%, by mass %, and T.sub.CS1 is a crystallization temperature in the composition.

10. A method for continuously casting Al-containing hypo-peritectic steel, the method comprising: supplying the mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 2 into a mold for continuous casting when Al-containing hypo-peritectic steel having Al: 0.2% to 2.0%, by mass %, and, in a hypo-peritectic region, C: 0.08% to 0.17%, by mass %, is continuously cast, wherein a slab drawing speed is in a range of 0.7 m/min to 2.0 m/min, and a thickness of a mold powder molten layer is in a range of 8×Q.sup.1/2 mm to 18×Q.sup.1/2 mm for a casting flow rate (Q; tons/min) of molten steel and is 35 mm or less.

11. A method for continuously casting Al-containing hypo-peritectic steel, the method comprising: supplying the mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 3 into a mold for continuous casting when Al-containing hypo-peritectic steel having Al: 0.2% to 2.0%, by mass %, and, in a hypo-peritectic region, C: 0.08% to 0.17%, by mass %, is continuously cast, wherein a slab drawing speed is in a range of 0.7 m/min to 2.0 m/min, and a thickness of a mold powder molten layer is in a range of 8×Q.sup.1/2 mm to 18×Q.sup.1/2 mm for a casting flow rate (Q; tons/min) of molten steel and is 35 mm or less.

12. A method for continuously casting Al-containing hypo-peritectic steel, the method comprising: supplying the mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 4 into a mold for continuous casting when Al-containing hypo-peritectic steel having Al: 0.2% to 2.0%, by mass %, and, in a hypo-peritectic region, C: 0.08% to 0.17%, by mass %, is continuously cast, wherein a slab drawing speed is in a range of 0.7 m/min to 2.0 m/min, and a thickness of a mold powder molten layer is in a range of 8×Q.sup.1/2 mm to 18×Q.sup.1/2 mm for a casting flow rate (Q; tons/min) of molten steel and is 35 mm or less.

13. A method for continuously casting Al-containing hypo-peritectic steel, the method comprising: supplying the mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 6 into a mold for continuous casting when Al-containing hypo-peritectic steel having Al: 0.2% to 2.0%, by mass %, and, in a hypo-peritectic region, C: 0.08% to 0.17%, by mass %, is continuously cast, wherein a slab drawing speed is in a range of 0.7 m/min to 2.0 m/min, and a thickness of a mold powder molten layer is in a range of 8×Q.sup.1/2 mm to 18×Q.sup.1/2 mm for a casting flow rate (Q; tons/min) of molten steel and is 35 mm or less.

14. A method for continuously casting Al-containing hypo-peritectic steel, the method comprising: supplying the mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 7 into a mold for continuous casting when Al-containing hypo-peritectic steel having Al: 0.2% to 2.0%, by mass %, and, in a hypo-peritectic region, C: 0.08% to 0.17%, by mass %, is continuously cast, wherein a slab drawing speed is in a range of 0.7 m/min to 2.0 m/min, and a thickness of a mold powder molten layer is in a range of 8×Q.sup.1/2 mm to 18×Q.sup.1/2 mm for a casting flow rate (Q; tons/min) of molten steel and is 35 mm or less.

15. A method for continuously casting Al-containing hypo-peritectic steel, the method comprising: supplying the mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 8 into a mold for continuous casting when Al-containing hypo-peritectic steel having Al: 0.2% to 2.0%, by mass %, and, in a hypo-peritectic region, C: 0.08% to 0.17%, by mass %, is continuously cast, wherein a slab drawing speed is in a range of 0.7 m/min to 2.0 m/min, and a thickness of a mold powder molten layer is in a range of 8×Q.sup.1/2 mm to 18×Q.sup.1/2 mm for a casting flow rate (Q; tons/min) of molten steel and is 35 mm or less.

16. A method for continuously casting Al-containing hypo-peritectic steel, the method comprising: supplying the mold powder for continuous casting of Al-containing hypo-peritectic steel according to claim 9 into a mold for continuous casting when Al-containing hypo-peritectic steel having Al: 0.2% to 2.0%, by mass %, and, in a hypo-peritectic region, C: 0.08% to 0.17%, by mass %, is continuously cast, wherein a slab drawing speed is in a range of 0.7 m/min to 2.0 m/min, and a thickness of a mold powder molten layer is in a range of 8×Q.sup.1/2 mm to 18×Q.sup.1/2 mm for a casting flow rate (Q; tons/min) of molten steel and is 35 mm or less.

Description

DETAILED DESCRIPTION

[0035] The disclosed embodiments are described below in detail.

[0036] In continuous casting of steel, a mold powder is added to the surface of molten steel in a mold for continuous casting. The mold powder added into the mold is heated by the heat of the molten steel in the mold to have a temperature gradient in which the temperature is high on the side of the mold powder that is in contact with the molten steel in the mold and the temperature is low on the opposite side of the mold powder that is in contact with air. That is, the mold powder on the surface side of the molten steel in the mold is melted to form a molten mold powder layer (referred to as the “mold powder molten layer”) on the surface of the molten steel in the mold. On the mold powder molten layer, a mold powder layer (referred to as the “mold powder semi-molten layer”) in which a molten layer and a solid layer are present together is formed. On the mold powder semi-molten layer, a mold powder layer (referred to as the “mold powder solid layer”) in which, although a portion of C (carbon) contained therein is burned, other components are substantially the same as those in an initial composition is formed. Herein, “the initial composition of the mold powder” is the composition of the mold powder before being added into the mold.

[0037] The molten mold powder flows into a space between the mold and a solidified shell and is consumed. Therefore, the molten mold powder is supplied from the mold powder semi-molten layer to the mold powder molten layer so as to compensate for the consumed mold powder. Furthermore, the fresh mold powder is added onto the mold powder solid layer so as to compensate for the consumed mold powder. In this manner, the mold powder functions as a lubricant between the solidified shell and the mold, an oxidation inhibitor for the molten steel in the mold, and a heat insulator.

[0038] In continuous casting of Al-containing hypo-peritectic steel which contains 0.2% by mass to 2.0% by mass Al and which has a carbon content in a hypo-peritectic region (0.08% by mass to 0.17% by mass), the composition of the mold powder molten layer is varied by the reaction of the mold powder molten layer with Al in molten steel. Since the composition of the mold powder molten layer is varied, high-melting point crystals such as gehlenite (2CaO.Al.sub.2O.sub.3.SiO.sub.2) are formed.

[0039] The disclosed embodiments include a technique that is intended to suppress the formation of high-melting point crystals such as gehlenite and to stably and homogeneously precipitate cuspidine (2SiO.sub.2.3CaO.CaF.sub.2) in the continuous casting of Al-containing hypo-peritectic steel even if the composition of the mold powder molten layer is varied as described above. The stable, homogeneous precipitation of cuspidine enables a slow cooling effect due to the mold powder to be maintained.

[0040] The mold powder according to the disclosed embodiments contains, as basic components, CaO (calcium oxide), SiO.sub.2 (silicon oxide), Na.sub.2O (sodium oxide), Li.sub.2O (lithium oxide), F (fluorine), and C (carbon) and the control range of those components is controlled as described below.

[0041] First, “CaO content (mass percent)/SiO.sub.2 content (mass percent)” in the initial composition of the mold powder is set to greater than or equal to “1.0+0.05×[Al content (mass percent) of molten steel]” and less than or equal to “2.0−0.35×[Al content (mass percent) of molten steel]”. Herein, [Al content (mass percent) of molten steel] is the Al content of molten steel that is continuously cast. Thus, when the Al content of molten steel that is continuously cast is, for example, 1.0% by mass, “CaO content (mass percent)/SiO.sub.2 content (mass percent)” of the mold powder needs to be within the range of 1.05 to 1.65.

[0042] As described above, SiO.sub.2 in the mold powder molten layer is reduced by Al in molten steel and is reduced in amount. Therefore, the basicity ((mass percent CaO)/(mass percent SiO.sub.2)) of the mold powder molten layer increases from the initial stage to later stage of continuous casting. This change is promoted as the Al content of molten steel is higher.

[0043] Thus, “CaO content (mass percent)/SiO.sub.2 content (mass percent)” in the initial composition of the mold powder, that is, the basicity ((mass percent CaO)/(mass percent SiO.sub.2)) is adjusted depending on the Al content of molten steel that is continuously cast such that the minimum basicity of the precipitation region of cuspidine is ensured. In addition, the composition of the mold powder molten layer is designed so as to always overlap the precipitation region of cuspidine even if the basicity ((mass percent CaO)/(mass percent SiO.sub.2)) of the mold powder molten layer increases. At the same time, in this region, the precipitation of cuspidine and mayenite (12CaO.7Al.sub.2O.sub.3) can be maximally used even if the composition of the mold powder molten layer varies in the direction of enrichment of Al.sub.2O.sub.3 (aluminum oxide).

[0044] When “CaO content (mass percent)/SiO.sub.2 content (mass percent)” in the initial composition of the mold powder is greater than “2.0−0.35×[Al content (mass percent) of molten steel]”, the basicity ((mass percent CaO)/(mass percent SiO.sub.2)) of the mold powder molten layer in the later stage of continuous casting increases excessively and the crystallization temperature increases significantly. As a result, a continuous casting operation becomes unstable. In order to prevent this, “CaO content (mass percent)/SiO.sub.2 content (mass percent)” in the initial composition of the mold powder is adjusted to a lower value as the Al content of molten steel that is continuously cast is higher. Herein, the “crystallization temperature” is the temperature at which exothermic heat associated with crystal formation is measured when the mold powder completely melted at 1,300° C. in a platinum crucible is constantly cooled at a cooling rate of 5° C./min.

[0045] When “CaO content (mass percent)/SiO.sub.2 content (mass percent)” in the initial composition of the mold powder is less than 1.0, the crystallization temperature decreases to 1,100° C. or lower and the precipitation of crystals is suppressed. This allows the solidified shell to be strongly cooled, thereby causing longitudinal cracking on a surface of the solidified shell. Furthermore, when Al.sub.2O.sub.3 in the mold powder molten layer is enriched to reach the formation region of gehlenite, the viscosity of the mold powder molten layer increases sharply; hence, depressions or transverse cracks in a surface of a cast slab increase.

[0046] Even in a region in which “CaO content (mass percent)/SiO.sub.2 content (mass percent)” in the initial composition of the mold powder is greater than or equal to 1.0 and less than “1.0+0.05×[Al content (mass percent) of molten steel], a component range in which cuspidine crystals precipitate is partly present. However, in this region, when the mold powder molten layer absorbs Al.sub.2O.sub.3 suspended in molten steel and the Al.sub.2O.sub.3 content of the mold powder molten layer increases, the composition of the mold powder molten layer varies to the precipitation region of gehlenite. Therefore, there is a risk that the stability of a continuous casting operation decreases.

[0047] The above risk increases as the Al content of molten steel is higher. Therefore, as hypo-peritectic steel has a higher Al content, the lower limit of “CaO content (mass percent)/SiO.sub.2 content (mass percent)” in the initial composition of the mold powder is adjusted to a higher value so as to maintain a component range in which cuspidine always precipitates stably. Thus, in the disclosed embodiments, “CaO content (mass percent)/SiO.sub.2 content (mass percent)” in the initial composition of the mold powder is set to greater than or equal to “1.0+0.05×[Al content (mass percent) of molten steel]”.

[0048] In the disclosed embodiments, the amounts of blended Na.sub.2O, Li.sub.2O, and F are appropriately controlled for the purpose of maintaining the viscosity of the mold powder molten layer at a low level and promoting the melting of the mold powder and the uniform flow of the mold powder between the mold and the solidified shell. Details are as described below.

[0049] That is, the Na.sub.2O content in the initial composition of the mold powder is set to 8% by mass or less. Since Na has higher affinity to F than Ca, the excessive addition of Na.sub.2O inhibits the formation of cuspidine. Furthermore, when Na.sub.2O is excessively contained, nepheline (NaAlSiO.sub.4) is likely to precipitate and coarse cuspidine is likely to be non-uniformly formed with nepheline acting as a nucleus. As a result, a large slag bear is formed to induce the non-uniform flow of the mold powder between the mold and the solidified shell. This influence is significant when the Na.sub.2O content is more than 8% by mass; hence, the upper limit of the Na.sub.2O content is 8% by mass. From the viewpoint of promoting the homogeneous precipitation of fine cuspidine crystals to suppress the surface cracking of the cast slab, the Na.sub.2O content is preferably 5% by mass or less and more preferably 4% by mass or less. On the other hand, the lower limit of the Na.sub.2O content is not limited and the component range is determined depending on an appropriate blending ratio with Li.sub.2O and F described below.

[0050] Since the Na.sub.2O content is preferably low as described above, Li.sub.2O can be used, instead of Na.sub.2O, as a viscosity modifier and melting accelerator for the mold powder. In this case, when “Li.sub.2O content (mass percent)/Na.sub.2O content (mass percent)” in the initial composition of the mold powder is 0.6 or more, fine uniform cuspidine crystals can be stably formed. Here, when the Li.sub.2O content is less than 2% by mass, this effect is small. Therefore, the Li.sub.2O content is 2% by mass or more. On the other hand, when the Li.sub.2O content is more than 7% by mass, the precipitation of cuspidine is inhibited and production costs of the mold powder increase significantly. Therefore, the upper limit of the Li.sub.2O content is 7% by mass.

[0051] When “Li.sub.2O content (mass percent)/Na.sub.2O content (mass percent)” is more than 2.0, negative effects such as the inhibition of cuspidine precipitation and the significant increase in production costs of the mold powder appear in some cases. Thus, “Li.sub.2O content (mass percent)/Na.sub.2O content (mass percent)” is preferably 2.0 or less.

[0052] F (fluorine) is essential for the formation of cuspidine and has a large effect of suppressing an increase in viscosity when the Al.sub.2O.sub.3 content increases; hence, a certain amount or more of F is contained. However, the excessive addition of F relative to Na.sub.2O and Li.sub.2O excessively promote the melting of the mold powder to excessively increase the thickness of the mold powder molten layer. Therefore, as a result, the distance from the surface of the molten steel in the mold to the upper surface of the mold powder molten layer increases and the temperature of the mold powder molten layer decreases, thereby promoting the increase in viscosity of the mold powder molten layer and the coarsening of slag bear. This causes deep oscillation marks or depressions on a surface of the cast slab.

[0053] Therefore, in the initial composition of the mold powder, the Na.sub.2O content, the Li.sub.2O content, and the F content are adjusted to a range satisfying Inequality (1) below.

[00002]$\begin{array}{cc}10<\left({\mathrm{Li}}_{2}O\mathrm{content}\left(\mathrm{mass}\mathrm{percent}\right)\right)+0.5\times \left({\mathrm{Na}}_{2}O\mathrm{content}\left(\mathrm{mass}\mathrm{percent}\right)\right)+0.8\times \left(F\mathrm{content}\left(\mathrm{mass}\mathrm{percent}\right)\right)<20& \left(1\right)\end{array}$

[0054] Herein, when “(Li.sub.2O content (mass percent))+0.5×(Na.sub.2O content (mass percent))+0.8×(F content (mass percent))” is 10 or less, the viscosity of the initial composition of the mold powder is slightly high and the increase in viscosity thereof becomes large when the Al.sub.2O.sub.3 content of the mold powder molten layer increases. These deteriorate the uniform flow of the mold powder between the mold and the solidified shell. Thus, in the disclosed embodiments, “(Li.sub.2O content (mass percent))+0.5×(Na.sub.2O content (mass percent))+0.8×(F content (mass percent))” is more than 10 from the viewpoint of maintaining the low viscosity and uniform flow of the mold powder.

[0055] On the other hand, when “(Li.sub.2O content (mass percent))+0.5×(Na.sub.2O content (mass percent))+0.8×(F content (mass percent))” is 20 or more, slag forming properties of the mold powder are excessively good, and sintering of the mold powder and slag bear increase. In order to prevent these, in the disclosed embodiments, “(Li.sub.2O content (mass percent))+0.5×(Na.sub.2O content (mass percent))+0.8×(F content (mass percent))” is less than 20.

[0056] When F is contained excessively relative to Na.sub.2O or Li.sub.2O, though it is easy to obtain cuspidine crystals, the surface quality of the cast slab is impaired by the formation of slag bear or an increase in viscosity in association with the increase of the crystallization temperature. Therefore, in the initial composition of the mold powder, the Na.sub.2O content, the Li.sub.2O content, and the F content are adjusted to a range satisfying Inequality (2) below.

[00003]$\begin{array}{cc}1.00\le \left(F\mathrm{content}\left(\mathrm{mass}\mathrm{percent}\right)\right)/\left[\left({\mathrm{Li}}_{2}O\mathrm{content}\left(\mathrm{mass}\mathrm{percent}\right)\right)+0.5\times \left({\mathrm{Na}}_{2}O\mathrm{content}\left(\mathrm{mass}\mathrm{percent}\right)\right)+1.46\right]\ge 1.24& \left(2\right)\end{array}$

[0057] Herein, the Na.sub.2O content, the Li.sub.2O content, and the F content are adjusted so as to satisfy that “(F content (mass percent))/[(Li.sub.2O content (mass percent))+0.5×(Na.sub.2O content (mass percent))+1.46]” is 1.24 or less, thereby enabling the viscosity and the crystallization temperature to be appropriately adjusted. On the other hand, when “(F content (mass percent))/[(Li.sub.2O content (mass percent))+0.5×(Na.sub.2O content (mass percent))+1.46]” is less than 1.00, F combines with, for example, Na.sub.2O or Li.sub.2O, the amount of F necessary to form cuspidine decreases, and the formation of crystals decreases. Thus, “(F content (mass percent))/[(Li.sub.2O content (mass percent))+0.5×(Na.sub.2O content (mass percent))+1.46]” is 1.00 or more.

[0058] C (carbon) is a component which functions as a melting rate modifier for the mold powder and which is essential for the mold powder. When the C content is less than 2% by mass, the melting rate of the mold powder is excessively high. This leads to formation of aggregates in which the unmolten mold powder is caught and is solidified, causes the coarsening of slag bear and the engagement of contaminants, and causes the destabilization of a continuous casting operation. Thus, in the initial composition of the mold powder, the lower limit of the C content is 2% by mass.

[0059] On the other hand, when the carbon content is more than 10% by mass, the spread of the molten mold powder is suppressed excessively and therefore the risk of a breakout due to insufficient lubrication between the mold and the solidified shell increases. Thus, in the initial composition of the mold powder, the C content is 10% by mass or less.

[0060] Furthermore, in the initial composition of the mold powder, the composition may contain one or more of 5% by mass or less K.sub.2O, 5% by mass or less MnO, 5% by mass or less MgO, 5% by mass or less B.sub.2O.sub.3, and 5% by mass or less BaO. These components may be used as flux instead of Na.sub.2O and Li.sub.2O. However, the excessive addition of a solvent inhibits the precipitation of cuspidine to reduce the crystallization temperature and causes the coarsening of slag bear due to the excessive melting of the mold powder as described above. Thus, the sum of the contents of these components is preferably 5% by mass or less and more preferably 3% by mass or less. The addition of B.sub.2O.sub.3 causes the movement of B (boron) from the mold powder molten layer to molten steel, increases the B content of molten steel to cause the hardening and embrittlement of the solidified shell, and causes the deterioration in surface quality of the cast slab. Therefore, the amount of added B.sub.2O.sub.3 is preferably less than 2% by mass.

[0061] In the initial composition of the mold powder, the content of Al.sub.2O.sub.3 is preferably low. In the initial composition of the mold powder, the less the content of Al.sub.2O.sub.3 is, the more the mold powder molten layer can be maintained in a component range in which the precipitation of cuspidine can be used and changes in properties of the mold powder molten layer can be stabilized at a low level. Therefore, in the initial composition of the mold powder, the content of Al.sub.2O.sub.3 is preferably 3% by mass or less and more preferably 2% by mass or less.

[0062] Adjustment to the above composition range allows the mold powder for continuous casting of Al-containing hypo-peritectic steel according to the disclosed embodiments to be controlled to have characteristic properties below. The range and purpose of properties are described below.

[0063] Initial properties of the mold powder are preferably controlled to a range below in view of changes in properties due to the enrichment of Al.sub.2O.sub.3.

[0064] The crystallization temperature of the mold powder is 1,100° C. to 1,250° C. When the crystallization temperature is lower than 1,100° C., a slow cooling effect is insufficient and longitudinal cracking occurs on a surface of the cast slab. However, when the crystallization temperature is higher than 1,250° C., there is a risk of a breakout because the crystallization temperature is excessively high and the fluidity of the mold powder is inhibited.

[0065] The viscosity of the mold powder at 1,300° C. is 0.05 Pa.Math.s to 0.20 Pa.Math.s. When the viscosity at 1,300° C. is less than 0.05 Pa.Math.s, scab defects due to the mold powder may possibly occur in steel products because the mold powder molten layer is incorporated in molten steel by a turbulent flow on the surface of the molten steel in the mold and adheres to an inner layer of the solidified shell. However, when the viscosity at 1,300° C. is more than 0.20 Pa.Math.s, the insufficient flow or non-uniform flow of the mold powder between the mold and the solidified shell is caused and furthermore the formation of slag bear is caused because the maximum viscosity is excessively high when the viscosity of the mold powder molten layer increases in association with the enrichment of AlO.sub.3. These cause a breakout or the surface cracking of the cast slab.

[0066] When changes in properties in association with the enrichment of Al.sub.2O.sub.3 are large, variations in properties of the mold powder molten layer increase to destabilize a continuous casting operation. Therefore, the increment of the crystallization temperature and the increment of the viscosity are preferably suppressed to a low level. In the mold powder according to the disclosed embodiments, the Na.sub.2O content, the Li.sub.2O content, and the F content are adjusted to the above ranges, whereby even if the reduction in amount of SiO.sub.2 and the enrichment of AlO.sub.3 occur with respect to the initial composition of the mold powder, the changes of the crystallization temperature and the viscosity in association with these changes are suppressed. This is a feature of the mold powder according to the disclosed embodiments.

[0067] The viscosity in the initial composition of the mold powder at 1,300° C. is represented by η.sub.0 and the crystallization temperature in the initial composition is represented by T.sub.CS0. Supposing that SiO.sub.2 of the initial composition of the mold powder is reduced by Al, the viscosity of the mold powder molten layer at 1,300° C. in a composition in which the SiO.sub.2 content is reduced by 17.6% by mass from the initial composition and the Al.sub.2O.sub.3 content is increased by 20.0% by mass from the initial composition is represented by η.sub.1 and the crystallization temperature in this composition is represented by T.sub.CS1.

[0068] In the mold powder of the above composition according to the disclosed embodiments, viscosity difference (Δη=η.sub.1−η.sub.0) between the viscosity η.sub.1 and the viscosity η.sub.0 is controlled to 0.15 Pa.Math.s or less and crystallization temperature difference (ΔT.sub.CS=T.sub.CS1−T.sub.CS0) between the crystallization temperature T.sub.CS1 and crystallization temperature T.sub.CS0 is controlled to 100° C. or lower.

[0069] When the changes in viscosity and crystallization temperature of the mold powder molten layer are larger than the above, the crystallization behavior of the mold powder and the flow behavior of the mold powder between the mold and the solidified shell rapidly vary due to the enrichment of Al.sub.2O.sub.3 and variations depending on places in the mold increase. As a result, surface defects of the cast slab are not able to be prevented and the risk of a breakout increases.

[0070] Herein, the viscosity of the mold powder was measured by a platinum ball draw-up method after the mold powder was charged into a platinum crucible and was completely melted by heating to 1,300° C. in a ring furnace. In this measurement, the temperature of the mold powder was measured with a thermocouple placed on an outer surface layer of the platinum crucible, and was calibrated with the difference from the inside temperature of the crucible which was determined in advance. The temperature of the molten mold powder was measured in such a manner that the platinum crucible containing the molten mold powder was cooled at a cooling rate of 5° C./min in terms of furnace body temperature. The temperature at which the cooling rate of the mold powder fell below the cooling rate of the furnace body temperature was taken as the exothermic onset temperature associated with crystal formation, which was defined as the crystallization temperature.

[0071] Casting conditions in a continuous casting method according to embodiments using the mold powder according to the disclosed embodiments that has the above composition and properties are described below.

[0072] The mold powder according to the disclosed embodiments is applied to Al-containing hypo-peritectic steel which contains 0.2% by mass to 2.0% by mass Al and which has a carbon content in a hypo-peritectic region (0.08% by mass to 0.17% by mass). In the case of Al-containing hypo-peritectic steel with an Al content of more than 2.0% by mass, it is very difficult to maintain changes in properties due to the enrichment of Al.sub.2O.sub.3 within a predetermined range. On the other hand, Al-containing hypo-peritectic steel with an Al content of less than 0.2% by mass can be dealt with a conventional mold powder for continuous casting of hypo-peritectic steel. Of course, the mold powder according to the disclosed embodiments may be used to continuously cast Al-containing hypo-peritectic steel with an Al content of less than 0.2% by mass.

[0073] The slab drawing speed is preferably 0.7 m/min to 2.0 m/min. When the slab drawing speed is less than 0.7 m/min, the fluidity of the mold powder molten layer is extremely poor and the surface quality of the cast slab deteriorates because the supply of heat to the mold powder added onto the surface of the molten steel in the mold is insufficient and the mold powder is not sufficiently melted. However, when the slab drawing speed is more than 2.0 m/min, the amount of the mold powder flowing between the mold and the solidified shell is insufficient and there is a risk of a breakout.

[0074] In relation to the above, it is preferable that, as an indicator for appropriately melting the mold powder, the thickness of the mold powder molten layer is 8×Q.sup.1/2 mm to 18×Q.sup.12 mm for the casting flow rate (Q; tons/min) of molten steel and is 35 mm or less.

[0075] Herein, the molten steel casting flow rate Q is calculated as “Q=7,800 (kg/m.sup.3)×cast slab width (m)×cast slab thickness (m)×slab drawing speed (m/min)/10.sup.3”. The molten steel casting flow rate Q relates to the supply of heat to the mold powder on the surface of the molten steel in the mold and is an important indicator for stably melting the mold powder to allow the molten mold powder to flow between the mold and the solidified shell.

[0076] When the thickness of the mold powder molten layer is less than 8×Q.sup.1/2 mm, the melting rate of the mold powder is insufficient as compared to the consumption thereof and the risk of a breakout due to insufficient lubrication between the mold and the solidified shell increases. In addition, the amount of the mold powder locally flowing between the mold and the solidified shell is likely to vary due to the change in level of the molten steel in the mold, thereby causing longitudinal cracking on a surface of the cast slab.

[0077] However, when the thickness of the mold powder molten layer is more than 18×Q.sup.1/2 mm or more than 35 mm, the distance between the upper surface (particularly the vicinity of the mold) of the mold powder molten layer and the surface of the molten steel in the mold increases, the temperature of the mold powder molten layer decreases, and therefore an increase in viscosity or the formation of slag bear is caused. This induces the occurrence of the surface cracking of the cast slab or a breakout.

[0078] As described above, according to the mold powder for continuous casting of Al-containing hypo-peritectic steel and continuous casting method of the disclosed embodiments, the occurrence of longitudinal cracking, transverse cracking, corner cracking, and depressions on a surface of a continuously cast slab of hypo-peritectic steel can be prevented. This enables a continuously cast slab of Al-containing hypo-peritectic steel having an Al content of 0.2% by mass to 2.0% by mass and a carbon content in a hypo-peritectic region, the continuously cast slab being excellent in surface quality, to be stably manufactured.

Examples

[0079] In order to confirm effects of the disclosed embodiments, the crystallization behavior of mold powders was confirmed and a continuous casting test for Al-containing hypo-peritectic steel was carried out.

[0080] The compositions of various mold powders that were tested are shown in Table 1. In Table 1, the viscosity and crystallization temperature in the initial composition of each mold powder at 1,300° C. are shown as initial properties. In addition, the viscosity and crystallization temperature at 1,300° C. in a simulated composition supposed that SiO.sub.2 in a mold powder is reduced by Al in molten steel, the SiO.sub.2 content of the mold powder is reduced by 17.6% by mass from the initial composition, and the Al.sub.2O.sub.3 content is increased by 20.0% by mass from the initial composition are shown.

TABLE-US-00001 TABLE 1 Mold powder Initial composition ratio Li.sub.2O + F/(Li.sub.2O + Initial composition (mass percent) CaO/ Li.sub.2O/ 0.5Na.sub.2O + 0.5Na.sub.2O + Level SiO.sub.2 Al.sub.2O.sub.3 CaO Na.sub.2O F Li.sub.2O K.sub.2O MnO MgO BaO B.sub.2O.sub.3 C SiO.sub.2 Na.sub.2O 0.8F 1.46 A1 35.9 1.3 37.9 3.1 10.4 5.4 — — 0.7 — — 5.3 1.06 1.74 15.30 1.237 A2 33.7 1.0 43.6 3.6 8.7 3.9 — — 0.6 — — 4.7 1.29 1.08 12.69 1.215 A3 35.0 0.7 38.8 3.6 10.0 5.0 1.1 — 0.5 0.5 — 4.8 1.11 1.39 14.83 1.211 A4 34.0 0.5 40.0 4.1 10.4 5.4 — — 0.6 — — 5.0 1.18 1.32 15.80 1.167 A5 33.7 0.6 40.1 5.0 9.9 5.2 — — 0.5 — — 5.0 1.19 1.04 15.65 1.081 A6 31.6 0.5 37.6 6.1 12.0 5.4 — — 0.6 — 2.0 4.3 1.19 0.89 18.09 1.211 A7 28.9 1.8 42.5 4.0 11.5 6.3 — — 0.6 — — 4.4 1.47 1.58 17.53 1.178 A8 27.0 1.5 42.9 4.1 12.0 6.2 0.5 — 0.7 — — 5.1 1.59 1.51 17.89 1.236 A9 30.0 1.5 40.3 4.3 10.5 5.6 — — 2.6 — — 5.2 1.34 1.30 16.18 1.140  A10 30.7 0.8 40.8 4.5 11.0 5.5 — — — 2.0 — 4.7 1.33 1.22 16.58 1.194  A11 30.1 6.0 40.1 4.1 10.2 5.4 — — — — — 4.1 1.33 1.32 15.64 1.145  A12 35.0 0.6 36.1 3.9 9.8 5.0 — 3.1 1.0 — — 5.4 1.03 1.28 14.79 1.165  A13 31.0 1.5 41.3 4.7 10.3 4.6 — 0.5 0.7 — 0.5 5.0 1.33 0.98 15.19 1.225 B1 39.3 1.7 23.0 13.0 12.2 5.0 — — 0.4 — — 5.3 0.59 0.38 21.30 0.941 B2 39.5 1.7 34.2 5.2 9.9 4.0 — — 0.4 — — 5.2 0.87 0.77 14.55 1.228 B3 33.5 5.5 42.1 3.1 7.5 2.2 — — 0.6 — — 5.5 1.26 0.71 9.77 1.440 B4 30.4 5.2 38.4 5.5 11.0 3.3 — — 0.6 — — 5.5 1.26 0.60 14.88 1.465 B5 33.4 0.8 39.1 4.3 8.9 6.7 — — 0.6 — 1.1 5.0 1.17 1.56 16.00 0.863 B6 31.7 2.2 39.0 10.0 11.8 0.0 — — 0.3 — — 4.9 1.23 0.00 14.48 1.827 B7 28.5 2.9 46.8 5.5 11.0 0.0 — — 0.5 — — 4.9 1.64 0.00 11.58 2.613 B8 30.2 0.7 41.0 4.7 14.1 2.8 — — 0.6 — — 5.9 1.36 0.60 16.47 2.133 B9 27.4 1.5 35.5 7.9 14.0 6.0 — — 0.6 — — 7.2 1.30 0.76 21.19 1.227  B10 27.1 2.0 42.5 10.5 11.0 2.5 — — 0.5 — — 4.0 1.57 0.24 16.58 1.194  B11 23.8 0.5 48.9 4.2 11.1 6.2 — — 0.5 — — 4.8 2.05 1.48 17.21 1.137  B12 21.7 4.7 42.9 3.0 8.6 8.0 — — 0.6 — — 10.5 1.98 2.67 16.41 0.785  B13 31.5 1.0 39.2 2.0 8.4 4.5 6.1 — 0.8 — — 6.3 1.24 2.25 12.25 1.207  B14 32.2 0.5 39.1 2.3 8.7 5.2 — — 7.0 — — 5.5 1.21 2.26 13.34 1.114  B15 30.3 0.6 38.0 3.0 8.4 5.0 — — 0.5 — 9.0 5.0 1.25 1.67 13.25 1.055  B16 33.6 0.5 36.6 2.5 7.0 3.0 — — 0.6 10.5  — 5.7 1.09 1.20 9.87 1.226 Mold powder At enrichment of Al.sub.2O.sub.3 Initial properties by 20.0% by mass Crystallization Crystallization Changes in Viscosity η.sub.0 temperature Viscosity η.sub.1 temperature properties at 1,300° C. T.sub.CS0 at 1,300° C. T.sub.CS1 Δη ΔT.sub.CS Level (Pa .Math. s) (° C.) (Pa .Math. s) (° C.) (Pa .Math. s) (° C.) Remarks A1 0.09 1144 0.15 1170 0.06 26 Example A2 0.08 1136 0.14 1145 0.06 9 Example A3 0.06 1125 0.10 1130 0.04 5 Example A4 0.07 1131 0.15 1140 0.08 9 Example A5 0.09 1136 0.15 1146 0.06 10 Example A6 0.05 1125 0.09 1130 0.04 5 Example A7 0.06 1166 0.13 1176 0.07 10 Example A8 0.06 1170 0.12 1195 0.06 25 Example A9 0.06 1145 0.11 1152 0.05 7 Example  A10 0.05 1132 0.09 1136 0.04 4 Example  A11 0.09 1133 0.21 1188 0.12 55 bxample  A12 0.07 1128 0.14 1150 0.07 22 Example  A13 0.06 1122 0.12 1136 0.06 14 Example B1 0.19 880 0.36 1023 0.17 143 Comparative example B2 0.15 989 0.38 1156 0.23 167 Comparative example B3 0.11 1071 0.30 1225 0.19 154 Comparative example B4 0.09 1203 0.25 1311 0.16 108 Comparative example B5 0.09 1100 0.23 1222 0.14 122 Comparative example B6 0.08 1188 0.25 1290 0.17 102 Comparative example B7 0.06 1261 0.24 1374 0.18 113 Comparative example B8 0.06 1285 0.22 1345 0.16 60 Comparative example B9 0.05 1103 0.12 1189 0.07 86 Comparative example  B10 0.06 1135 0.15 1264 0.09 129 Comparative example  B11 0.05 1030 0.11 1224 0.06 194 Comparative example  B12 0.06 1045 0.11 1230 0.05 185 Comparative example  B13 0.05 1090 0.19 1168 0.14 78 Comparative example  B14 0.06 1050 0.18 1145 0.12 95 Comparative example  B15 0.07 1078 0.15 1100 0.08 22 Comparative example  B16 0.14 990 0.31 1050 0.17 60 Comparative example

[0081] As is clear from Table 1, in Levels A1 to A13 that meet the composition range of a mold powder according to the disclosed embodiments, changes in viscosity and crystallization temperature in association with the enrichment of Al.sub.2O.sub.3 are suppressed. In contrast to this, in Levels B1 to B16 that are outside the scope of the disclosed embodiments, a significant increase in viscosity or crystallization temperature is observed and it is clear that the stability of properties is low.

[0082] Results of a continuous casting test carried out using the mold powders shown in Table 1 are described below. In the continuous casting test, about 270 tons of three types of molten steels (Steels 1 to 3) having a steel chemical composition shown in Table 2 were continuously cast under casting conditions shown in Table 3 using a vertical bending continuous casting machine. In the continuous casting test, the thickness of a cast slab was 250 mm, the width of the cast slab was 1250 mm, mold-oscillation conditions included a sinusoidal waveform with an amplitude of 3.5 mm (=a stroke of 7.0 mm), and the slab drawing speed was basically 1.3 m/min and was varied from 0.6 m/min to 2.2 m/min.

[0083] Each powdery mold powder with a composition shown in Table 1 was periodically and uniformly supplied to the surface of molten steel in a mold such that the consumption of the mold powder was in the range of 0.4 kg/m.sup.2 to 0.8 kg/m.sup.2. The thickness of a mold powder molten layer was measured three times in steady casting in which a cast slab with a length of about 40 m was continuously cast from the start of casting. The average thereof was regarded as the typical thickness of the mold powder molten layer.

[0084] The cast slab drawn from a mold for continuous casting was intermediately cooled in a secondary cooling zone and was cooled in an upper bending zone and a lower reformation zone under such cooling conditions that the corner temperature of the cast slab as estimated from two-dimensional heat transfer calculation avoided a brittle temperature zone at each steel chemical composition. In each continuous casting test, 12 steady casting zone cast slabs (slab cast slabs) with a predetermined length (about 9 m) were manufactured. The scope of the disclosure is not intended to be limited to the above manufacturing conditions.

TABLE-US-00002 TABLE 2 Chemical composition of steel (mass percent) C Si Mn P S sol. Al Cr Nb Mo N Steel 1 0.110 0.20 1.85 0.012 0.0030 0.45 0.15 — — 0.0030 Steel 2 0.155 0.22 1.56 0.009 0.0020 0.86 — 0.015 — 0.0029 Steel 3 0.133 0.31 1.80 0.004 0.0009 1.30 0.21 — 0.04 0.0033

TABLE-US-00003 TABLE 3 Casting conditions Mold CaO/SiO.sub.2 in initial Thickness Width Slab drawing powder composition of Steel Min. Max. of slab of slab speed Level used mold powder component CaO/SiO.sub.2 CaO/SiO.sub.2 (mm) (mm) (m/min) 1 A1 1.06 Steel 1 1.02 1.84 250 1250 1.3 2 A1 1.06 Steel 2 1.04 1.70 250 1250 1.3 3 A1 1.06 Steel 3 1.07 1.55 250 1250 1.3 4 A2 1.29 Steel 1 1.02 1.84 250 1250 1.3 5 A3 1.11 Steel 2 1.04 1.70 250 1250 1.3 6 A3 1.11 Steel 3 1.07 1.55 250 1250 1.3 7 A4 1.18 Steel 2 1.04 1.70 250 1250 1.3 8 A4 1.18 Steel 2 1.04 1.70 250 1250 0.6 9 A4 1.18 Steel 2 1.04 1.70 250 1250 2.2 10 A5 1.19 Steel 2 1.04 1.70 250 1250 1.3 11 A6 1.19 Steel 2 1.04 1.70 250 1250 1.3 12 A7 1.47 Steel 2 1.04 1.70 250 1250 1.3 13 A8 1.59 Steel 1 1.02 1.84 250 1250 1.3 14 A8 1.59 Steel 2 1.04 1.70 250 1250 1.3 15 A8 1.59 Steel 3 1.07 1.55 250 1250 1.3 16 A9 1.34 Steel 2 1.04 1.70 250 1250 1.3 17 A10 1.33 Steel 2 1.04 1.70 250 1250 1.3 18 A11 1.33 Steel 2 1.04 1.70 250 1250 1.3 19 A12 1.03 Steel 1 1.02 1.84 250 1250 1.3 20 A13 1.31 Steel 2 1.04 1.70 250 1250 1.3 21 B1 0.59 Steel 3 1.07 1.55 250 1250 1.3 22 B2 0.87 Steel 1 1.02 1.84 250 1250 1.3 23 B2 0.87 Steel 2 1.04 1.70 250 1250 1.3 24 B2 0.87 Steel 3 1.07 1.55 250 1250 1.3 25 B3 1.26 Steel 2 1.04 1.70 250 1250 1.3 26 B4 1.26 Steel 1 1.02 1.84 250 1250 1.3 27 B4 1.26 Steel 2 1.04 1.70 250 1250 1.3 28 B5 1.17 Steel 2 1.04 1.70 250 1250 1.3 29 B6 1.23 Steel 3 1.07 1.55 250 1250 1.3 30 B7 1.64 Steel 2 1.04 1.70 250 1250 1.3 31 B8 1.36 Steel 3 1.07 1.55 250 1250 1.3 32 B9 1.30 Steel 3 1.07 1.55 250 1250 1.3 33 B10 1.57 Steel 2 1.04 1.70 250 1250 1.3 34 B11 2.05 Steel 1 1.02 1.84 250 1250 1.3 35 B11 2.05 Steel 2 1.04 1.70 250 1250 1.3 36 B12 1.98 Steel 2 1.04 1.70 250 1250 1.3 37 B13 1.24 Steel 2 1.04 1.70 250 1250 1.3 38 B14 1.21 Steel 2 1.04 1.70 250 1250 1.3 39 B15 1.25 Steel 2 1.04 1.70 250 1250 1.3 40 B16 1.09 Steel 2 1.04 1.70 250 1250 1.3 Casting conditions Surface quality of slab Average thickness Transverse of molten layer Longitudinal cracking- Acceptance of mold powder cracking corner cracking rate Level (mm) (cracks/slab) (cracks/slab) (%) Remarks 1 19 0 0 100 Example 2 20 0 0 100 Example 3 19 0 2 83 Example 4 22 0 0 100 Example 5 20 0 0 100 Example 6 21 0 0 100 Example 7 23 0 0 100 Example 8 10 0 3 75 Comparative example 9 22 1 4 67 Comparative example 10 22 0 0 100 Example 11 23 0 0 100 Example 12 24 0 0 100 Example 13 26 0 0 100 Example 14 22 0 0 100 Example 15 25 0 2 83 Example 16 21 0 0 100 Example 17 23 0 0 100 Example 18 22 1 0 92 Example 19 26 0 0 100 Example 20 24 0 0 100 Example 21 36 5 14 0 Comparative example 22 25 2 7 33 Comparative example 23 25 3 8 25 Comparative example 24 25 5 10 0 Comparative example 25 25 2 7 50 Comparative example 26 25 0 3 67 Comparative example 27 23 0 5 58 Comparative example 28 25 2 2 75 Comparative example 29 19 0 6 50 Comparative example 30 17 1 4 58 Comparative example 31 21 0 5 58 Comparative example 32 37 0 6 42 Comparative example 33 22 1 3 75 Comparative example 34 26 3 5 58 Comparative example 35 25 4 7 42 Comparative example 36 24 3 8 33 Comparative example 37 22 3 4 58 Comparative example 38 23 3 5 67 Comparative example 39 20 6 2 58 Comparative example 40 20 5 7 8 Comparative example

[0085] One of the 12 cast slabs manufactured as described above was sampled at random and was used as an investigation object. The whole of a cast slab longitudinal surface and the whole of a cast slab transverse surface were inspected by liquid penetrant testing (color check, an aqueous dye), whereby the number of longitudinal cracks and transverse cracks or corner cracks in each cast slab was investigated. The number of longitudinal cracks and transverse cracks or corner cracks with a length of 10 mm or more in a longitudinal or transverse direction of the cast slab was counted.

[0086] Even a cast slab having a longitudinal crack and a transverse crack or a corner crack was determined to be acceptable if such crack was a shallow surface crack removable at a grinder stock removal of 2 mm or less on a cast slab longitudinal surface and a cast slab transverse surface or at a grinder stock removal of 10 mm or less on a cast slab corner. The percentage of the number of acceptable cast slabs in the 12 cast slabs was classified as an acceptance rate.

[0087] These results are shown in Table 3 together in the form of cast slab surface quality.

[0088] In a case where casting was performed using a mold powder composition and casting conditions within the scope of disclosed embodiments (Levels 1 to 7 and Levels 10 to 20), the occurrence of longitudinal cracks and transverse cracks or corner cracks was extremely rare and the acceptance rate of cast slabs was ensured at 80% or more. On the other hand, in a case where, though a mold powder composition was within the scope of disclosed embodiments, casting conditions were outside the scope of disclosed embodiments, (Levels 8 and 9), the uniform flow of a mold powder between a mold and a solidified shell was inhibited, deep depressions increased, and the occurrence of transverse cracking particularly increased due to the influence thereof.

[0089] In a case where the composition of a mold powder was outside the scope of disclosed embodiments (Levels 21 to 40), the mold powder had a low slow cooling effect and longitudinal cracking occurred frequently on a surface of a cast slab. In a mold powder with a poor component balance between Na.sub.2O, F, and Li.sub.2O, the occurrence of transverse cracking due to the formation of deep depressions increased.