MOLD FLUX FOR CONTINUOUS-CASTING STEEL

Abstract

Provided is mold flux that can prevent longitudinal cracks from forming on a surface of a slab upon continuous-casting hypo-peritectic steel, wherein CaO, SiO.sub.2, an alkali metal oxide and a fluorine compound are contained, 1.1≦(CaO).sub.h/(SiO.sub.2).sub.h≦1.9, 0.10≦(CaF.sub.2).sub.h/((CaO).sub.h+(SiO.sub.2).sub.h+(CaF.sub.2).sub.h)≦0.40 and 0≦(alkali metal fluoride).sub.h/((CaO).sub.h+(SiO.sub.2).sub.h+(alkali metal fluoride).sub.h)≦0.10 are satisfied, a solidification temperature is no less than 1300° C., and viscosity at 1450° C. is no more than 0.1 Pa.Math.s.

Claims

1. Mold flux used for continuous-casting hypo-peritectic steel whose C concentration is 0.08 to 0.18 mass %, wherein CaO, SiO.sub.2, an alkali metal oxide and a fluorine compound are contained, the following formulas (1), (2) and (3) are satisfied, and a solidification temperature is no less than 1300° C. and viscosity at 1450° C. is no more than 0.1 Pa.Math.s:
1.1≦f(1)≦1.9  (1)
0.10≦f(2)≦0.40  (2)
0≦f(3)≦0.10  (3)
wherein
f(1)=(CaO).sub.h/(SiO.sub.2).sub.h  (a)
f(2)=(CaF.sub.2).sub.h/((CaO).sub.h+(SiO.sub.2).sub.h+(CaF.sub.2).sub.h)  (b)
f(3)=(alkali metal fluoride).sub.h/((CaO).sub.h+(SiO.sub.2).sub.h+(alkali metal fluoride).sub.h)   (c)
(CaO).sub.h=(W.sub.CaO−(CaF.sub.2).sub.h−0.718)  (A)
(SiO.sub.2).sub.h=W.sub.SiO2  (B)
(CaF.sub.2).sub.h=(W.sub.F−W.sub.Li2O×1.27−W.sub.Na2O×0.613−W.sub.K2O×0.403)×2.05  (C)
(alkali metal fluoride).sub.h=W.sub.Li2O×1.74+W.sub.Na2O×1.35+W.sub.K2O×1.23  (D) wherein W.sub.i is a mass concentration of a component i in the mold flux.

2. The mold flux for continuous-casting hypo-peritectic steel according to claim 1, wherein 0.1-10 mass % of MnO is further contained.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0034] FIG. 1 is a view showing the mold flux for continuous-casting steel of this invention.

DESCRIPTION OF EMBODIMENTS

[0035] In this invention, an object of preventing longitudinal cracks from forming on the surface of a slab when hypo-peritectic steel containing an alloying element such as Cu, Ni, Cr, Mo, Nb, V, Ti and B is continuous-cast is realized by: preparing CaO, SiO.sub.2, an alkali metal oxide and a fluorine compound within the optimum range, to keep the composition within the primary crystallization field of cuspidine; and setting the solidification temperature in 1300° C. or more and the viscosity at 1450° C. in 0.1 Pa.Math.s or less. Preferably, the mold flux for continuous-casting steel according to this invention further contains 0.1 to 10 mass % of MnO.

[0036] Hereinafter the mold flux for continuous-casting steel of this invention (may be simply referred to as “the mold flux of this invention” below) will be explained. FIG. 1 is a view showing the mold flux for continuous-casting steel of this invention. As shown in FIG. 1, mold flux 1 of this invention is supplied to the surface of molten steel 4 that was poured into a mold 3 via an immersion nozzle 2. The mold flux 1 of this invention supplied in this way melts with heat supplied from the molten steel 4. After that, the mold flux 1 flows along the mold 3, and infiltrates into a gap between the mold 3 and a solidified shell 5, to form a film. The solidified shell 5, which is formed by cooling from the side of the mold 3 that is cooled by cooling means not shown, is withdrawn toward a lower part of the mold 3 with rolls 6, and is cooled by cooling water 7.

[0037] Patent Literature 1 formerly explained describes that if the solidification temperature is higher than 1250° C., the lubricity is disturbed and breakout cannot be prevented. While the proper viscosity of the mold flux is specified as 0.6 to 2.5 poises (=0.06 to 0.25 Pa.Math.s) at 1300° C. in the invention of Patent Literature 1, the viscosity in most of Examples illustrated in Patent Literature 1 is no less than 1 poise (=0.1 Pa.Math.s).

[0038] It is necessary to lessen the resistance when the solidified shell is withdrawn toward a lower part of the mold (frictional force in the mold) for keeping the lubricity in the mold upon continuous casting. Because the mold flux exists between the inner wall of the mold and the solidified shell, the above frictional force can be lessened by decreasing its viscosity.

[0039] There is a problem that when the viscosity of the mold flux is decreased, the mold flux tends to be involved in the molten steel in the mold, and droplets of the involved mold flux becomes non metallic inclusions in the vicinity of the surface of a slab, to make the cleanness deteriorate.

[0040] “Tetsu-to-Hagané”, Vol. 93 (2006), No. 5, page 362 describes that the involvement can be prevented by keeping the composition of mold flux having the basicity even if the viscosity at 1300° C. is as low as 1 poise (=0.1 Pa's) or less.

[0041] The present invention was made about mold flux used for continuous-casting hypo-peritectic steel whose C concentration is 0.08 to 0.18 mass % based on the above approaches.

[0042] In this invention, the basic components are CaO, SiO.sub.2 and a fluorine compound, which are components of cuspidine. In addition, an alkali metal oxide is added, so that the solidification temperature can be easy to be controlled comparably. Here, “basic components” are the components of cuspidine, which mean that the sum total of the concentrations of CaO, SiO.sub.2 and fluorine in a fluorine compound is no less than 60 mass %.

[0043] Each concentration of CaO, SiO.sub.2, a fluorine compound and an alkali metal oxide is adjusted so as to satisfy the following formulas (1), (2) and (3), which represent the conditions under which cuspidine is easy to be crystallized:

1.1≦f(1)≦1.9  (1)

0.10≦f(2)≦0.40  (2)

0≦f(3)≦0.10  (3)

wherein

f(1)=(CaO).sub.h/(SiO.sub.2).sub.h  (a)

f(2)=(CaF.sub.2).sub.h/((CaO).sub.h+(SiO.sub.2).sub.h+(CaF.sub.2).sub.h)  (b)

f(3)=(alkali metal fluoride).sub.h/((CaO).sub.h+(SiO.sub.2).sub.h+(alkali metal fluoride).sub.h)  (c)

(CaO).sub.h=(W.sub.CaO−(CaF.sub.2).sub.h−0.718)  (A)

(SiO.sub.2).sub.h=W.sub.SiO2  (B)

(CaF.sub.2).sub.h=(W.sub.F−W.sub.Li2O×1.27−W.sub.Na2O×0.613−W.sub.K2O×0.403)×2.05  (C)

(alkali metal fluoride).sub.h=W.sub.Li2O×1.74+W.sub.Na2O×1.35+W.sub.K2O×1.23  (D)

where W.sub.i is a mass concentration (mass %) of a component i in the mold flux.

[0044] The composition of the mold flux can be kept within the primary crystallization field of cuspidine by adjusting each concentration of CaO, SiO.sub.2, a fluorine compound and an alkali metal oxide so as to satisfy the conditions represented by the above formulas (1), (2) and (3).

[0045] Here, when f(1) does not reach 1.1 or when f(1) is over 1.9, the composition of the mold flux deviates from cuspidine. Thus, crystallization enough for mild cooling cannot be obtained. Therefore, in this invention, f(1) is 1.1 to 1.9. Preferably, f(1) is no less than 1.2 and especially preferably no less than 1.3 because it makes the composition of the mold flux more similar to cuspidine, which brings about the embodiment where effective crystallization is easy to be achieved. Further, in the same point of view, f(1) is preferably no more than 1.8, and especially preferably no more than 1.7. In this invention, the preferable range of f(1) is 1.2 to 1.8, and especially preferably 1.3 to 1.7.

[0046] When f(2) does not reach 0.10 or when f(2) is over 0.40, the composition of the mold flux deviates from cuspidine. Thus, crystallization enough for mild cooling cannot be obtained. Therefore, in this invention, f(2) is 0.10 to 0.40. Preferably, f(2) is no less than 0.12 and especially preferably no less than 0.15 because it makes the composition of the mold flux more similar to cuspidine, which brings about the embodiment where effective crystallization is easy to be achieved. Further, in the same point of view, f(2) is preferably no more than 0.35, and especially preferably no more than 0.30. In this invention, the preferable range of f(2) is 0.12 to 0.35, and especially preferably 0.15 to 0.30.

[0047] When f(3) does not reach 0 or when f(3) is over 0.10, the composition of the mold flux deviates from cuspidine. Thus, crystallization enough for mild cooling cannot be obtained. Therefore, in this invention, f(3) is 0 to 0.10. Preferably, f(3) is no more than 0.08 because it makes the composition of the mold flux more similar to cuspidine, which brings about the embodiment where effective crystallization is easy to be achieved.

[0048] Generally, the viscosity of mold flux at 1300° C. is used as the basis. However, in this invention where the composition is kept within the primary crystallization field of cuspidine whose melting point is 1410 to 1420° C., the mold flux is already solidified at 1300° C. by crystallization, and thus, the value at 1300° C. cannot be obtained. Thus, in this invention, the viscosity at 1450° C. is defined as 0.1 Pa.Math.s or below.

[0049] The lubricity can be kept with this viscosity even in the state where the solidification temperature is raised to 1300° C. or more, which has been conventionally said to be difficult. If the solidification temperature rises, an effect of mild cooling in the mold is improved according thereto. In this invention, it is impossible to raise the solidification temperature of the mold flux to or over the above described melting point of cuspidine.

[0050] Not a little Mn is added to hypo-peritectic steel to be practically used in order to improve the strength when used as a steel material. Thus, MnO generated by oxidation of Mn in the molten steel migrates to the mold flux during casting.

[0051] This MnO is a component that blocks crystallization of cuspidine. Thus, in this invention, preferably MnO was mixed in advance by amount corresponding to the MnO concentration which is increased by MnO migrating from the molten steel to the mold flux. According to this inventor's examination, the concentration of the contained MnO is no less than 0.1 mass % in view of achieving an embodiment where an effect by the addition is easily obtained. In addition, the concentration of the contained MnO is no more than 10 mass % in view of preventing such a case from happening that the solidification temperature of the mold flux is too much low not to obtain crystallization, which is necessary for mild cooling. In this invention, it is proper that the MnO concentration in the mold flux is set according to the Mn concentration in the molten steel.

[0052] In some cases, MgO, Al.sub.2O.sub.3, BaO, B.sub.2O.sub.3 and the like may be added in order to control physical properties of the mold flux such as the solidification temperature, the viscosity and the surface tension. However, it is better that their concentrations are low in order to promote crystallization of cuspidine. Their concentrations are desirably not beyond 7 mass % in total. When normal raw materials are used, the total concentration of them that are inevitably contained is about 2 to 5 mass %, and can be no more than 2 to 5 mass % by using artificial raw materials like a pre-melted base material.

EXAMPLES

[0053] The results of the experiments done for confirming effects of this invention will be described.

Example 1

[0054] Mold flux of this invention of each Example of this invention, Reference Example and Comparative Example shown in Tables 1 and 2 was made. Here, f(1), f(2) and f(3) shown in Table 2 below were indexes calculated from the above formulas (a), (b) and (c). Crystallization of cuspidine in the mold flux was able to be promoted by adjusting these indexes within the ranges of the above formulas (1), (2) and (3). Concerning the components whose mass concentrations (mass %) are not shown in Table 1 and 2, f(1), f(2) and f(3) were calculated assuming that the mass % was zero. It is difficult in a general chemical analysis to evaluate the concentration of no more than 0.1 mass % with high accuracy. Thus, in Tables 1 and 2, “<0.1” is shown if the concentration of a component was less than 0.1 mass %.

TABLE-US-00001 TABLE 1 Concentration of Component (mass %) Category Type SiO.sub.2 CaO Al.sub.2O.sub.3 MgO Na.sub.2O MnO F Example of A 30.6 55.1 4.2 0.7 <0.1 —* 9.5 This B 31.2 56.2 2.4 0.7 <0.1 —* 9.5 Invention C 30.1 51.2 4.0 0.6 3.5 —* 10.5 D 30.3 54.6 2.4 0.7 <0.1 2.5 9.5 E 33.7 53.9 2.3 0.7 <0.1 —* 9.5 F 35.7 56.3 2.1 0.6 <0.1 —* 5.3 G 34.1 54.9 2.5 1.0 1.0 —* 6.5 H 30.5 51.5 2.0 <0.1 4.5 —* 11.5 I 30.6 55.1 3.2 0.7 <0.1 1.0 9.5 J 33.7 53.9 2.0 0.5 <0.1 0.5 9.5 K 30.5 51.5 1.9 <0.1 1.0 0.1 11.5 Comparative a 28.8 48.9 3.3 0.5 6.0 1.5 11.0 Example b 27.0 48.7 3.8 0.6 8.0 —* 12.0 c 37.1 28.7 8.0 0.1 16.8 —* 9.5

TABLE-US-00002 TABLE 2 Index of Solidification This Invention Temperature Viscosity Category Type f(1) f(2) f(3) (° C.) (Pa .Math. s) Example of A 1.34 0.21 0 1311 0.07 This B 1.35 0.21 0 1355 0.07 Invention C 1.29 0.20 0.06 1302 0.05 D 1.34 0.22 0 1334 0.06 E 1.19 0.21 0 1301 0.07 F 1.36 0.11 0 1329 0.08 G 1.35 0.14 0.02 1332 0.07 H 1.27 0.21 0.08 1302 0.06 I 1.34 0.21 0 1306 0.06 J 1.19 0.21 0 1303 0.06 K 1.27 0.21 0.08 1301 0.07 Comparative a 1.33 0.18 0.11*  1255* 0.04 Example b 1.41 0.18 0.14*  1236* 0.03 c 0.8* 0* 0.25*  1060* 0.12*

[0055] All the compositions of the mold flux of Examples of this invention A to K satisfied the above formulas (1) to (3). Their solidification temperatures were no less than 1300° C., and the viscosities at 1450° C. were no more than 0.1 Pa.Math.s. In contrast, concerning the compositions of the mold flux of Comparative Examples a to c, the compositions of this mold flux did not satisfy any of the above formulas (1) to (3). As a result, at least one of the solidification temperature and the viscosity at 1450° C. of each thereof was out of the range of this invention. Values with marks of asterisks in Tables 1 and 2 represent being out of the range of this invention.

[0056] The mold flux of each Example of this invention A to K and Comparative Example a to c was used for continuous casting of hypo-peritectic steel having the composition shown in Table 3 below, containing Nb and Ti, and whose degree of superheat in a state of molten steel is high, and the results were compared. Here, a slab of 500 mm in width and 85 mm in thickness was produced with a vertical bending type continuous casting machine, using 2.5 tons of the molten steel, under the conditions where the casting speed was 1.0 m/min and the specific water flow of the secondary cooling water was 1.1 litter/kg.

TABLE-US-00003 TABLE 3 Concentration of Component (mass %) C Si Mn P S Nb Ti Al Remainder 0.09 0.15 1.40 0.010 0.005 0.0014 0.010 0.03 Fe and Impurities

[0057] The mold flux added to the inside of the mold was selectively used, and as to an effect of mild cooling, a solidification coefficient was obtained by calculating local heat flux in the mold, the surface temperature of the slab and the thickness of the solidified shell and its growth rate, to compare the obtained results. As to the lubricity, the frictional force in the mold was obtained, to compare the obtained results. The results are shown in Table 4.

[0058] The local heat flux in the mold was obtained from temperature measured with a thermocouple that was embedded at the center of the width of the longer side surface 35 mm under the meniscus, to be evaluated. The frictional force in the mold was obtained from difference in pressure of a hydraulic cylinder used for oscillation of the mold. The surface temperature of the slab was measured at the center of the width in the side of the inward curve of roll segments just before the curving point of the vertical bending type continuous casting machine. The thickness of the solidified shell and its growth rate were evaluated by such a method that: a FeS alloy was added to the molten steel in the mold at the time point just before the casting was ended, a portion of the slab where the alloy was added was cut in the casting direction, and the concentration distribution of S on the cut surface was transferred to photographic paper.

TABLE-US-00004 TABLE 4 Local Frictional Surface Temperature Length and Heat Flux Force in of Slab (Center of Solidification Number of in Mold Mold Width in Side of Coefficient Longitudinal Category Type (MW/m.sup.2) (N/mm.sup.2) Inward Curve) (° C.) (mm/min.sup.0.5) Cracks Example A 1.32 1.12 × 10.sup.−2 1150 11.2 None of This B 1.26 2.09 × 10.sup.−2 1140 12.3 None Invention C 1.41 1.14 × 10.sup.−2 1110 13.4 None D 1.29 1.19 × 10.sup.−2 1120 11.9 None E 1.42 1.13 × 10.sup.−2 1105 13.8 None F 1.39 1.53 × 10.sup.−2 1109 13.5 None G 1.34 1.47 × 10.sup.−2 1118 12.9 None H 1.41 1.06 × 10.sup.−2 1107 13.1 None I 1.34 1.10 × 10.sup.−2 1150 11.3 None J 1.44 1.09 × 10.sup.−2 1100 14.0 None K 1.42 1.04 × 10.sup.−2 1105 13.2 None Comparative a 1.48 1.26 × 10.sup.−2 1100 18.1 None Example b 1.71 1.04 × 10.sup.−2 1020 20.6 None c 1.93 0.87 × 10.sup.−2 950 24.8 100 mm × 2 cracks

[0059] From the results of the evaluation of the local heat flux in the mold, while the local heat flux was no less than 1.48 MW/m.sup.2 in every Comparative Example, it decreased to no more than 1.44 MW/m.sup.2 in every Example of this invention, from which an effect of mild cooling was confirmed.

[0060] The frictional force in the mold was no more than 2.09×10.sup.−2 (N/mm.sup.2) in every Example of this invention and Comparative Example. Thus, no problems occur to the lubricity, and normal oscillation marks were formed on the surface of the slab at regular intervals.

[0061] The results of the measurement of the surface temperature of the slab were as follows: the temperature in cases where the mold flux of Examples of this invention was used was equal to or higher than that in cases where the mold flux of Comparative Examples was used, from which an effect of mild cooling was confirmed.

[0062] From the results of the evaluation of the thickness of the solidified shell and its growth rate, while the solidification coefficient was 18.1 to 24.8 mm/min.sup.0.5 in every Comparative Example, it decreased to 11.2 to 14.0 mm/min.sup.0.5 in every Example of this invention, from which an effect of mild cooling was clearly confirmed on the growth of the solidified shell.

[0063] In every Example of this invention, the obtained slab had excellent surface properties and condition. No surface defects such as longitudinal cracks or depressions appeared. In contrast, in Comparative Example c, two longitudinal cracks of about 100 mm in length appeared at the center of the width.

Example 2

[0064] The mold flux of each Example of this invention A and Comparative Example a was used out of the mold flux tested in Example 1, and larger-scale continuous casting than Example 1 was carried out.

[0065] The molten steel of 300 tons, having the composition shown in Table 5, was supplied for casting using the mold flux of each Example, to cast twelve slabs of about 2300 mm in width, 300 mm in thickness and 6 m in length at the speed of 0.70 m/min. The results of the surfaces of the obtained slabs were as follows:

TABLE-US-00005 TABLE 5 Concentration of Component (mass %) C Si Mn P S Cr Mo Al Remainder 0.11 0.30 0.90 <0.025 <0.005 0.50 0.40 0.02 Fe and Impurities

[0066] From Example of this invention A, twelve slabs of excellent surfaces without any longitudinal cracks were obtained. These slabs were able to be supplied for a rolling process as they were.

[0067] In contrast, in Comparative Example a, longitudinal cracks formed on the surface of fours slabs, that is, the first, second, eleventh and twelfth slabs after casting was started. All the slabs where longitudinal clacks formed were necessary to be repaired (scarfing with a scarfer).

[0068] It is needless to say that this invention is not limited to the above described examples, and the embodiments may be properly modified if such modification is within the scope of the technical concepts recited in the claims.

REFERENCE SIGNS LIST

[0069] 1 . . . mold flux for continuous-casting steel [0070] 2 . . . immersion nozzle [0071] 3 . . . mold [0072] 4 . . . molten steel [0073] 5 . . . solidified shell [0074] 6 . . . rolls [0075] 7 . . . cooling water