Mold flux for continuous casting and continuous casting method
11453048 · 2022-09-27
Assignee
Inventors
Cpc classification
Y02P10/20
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
B22D11/108
PERFORMING OPERATIONS; TRANSPORTING
C22C38/002
CHEMISTRY; METALLURGY
International classification
B22D11/00
PERFORMING OPERATIONS; TRANSPORTING
Abstract
A mold flux for continuous casting has a base material composition containing 25 to 60 mass % of CaO, 15 to 45 mass % of SiO.sub.2, 5 to 25 mass % of F, 0.2 to 1.0 mass % of S, and 0 to 20 mass % of a total of Li.sub.2O, Na.sub.2O, and K.sub.2O, and in the base material composition, f(1) is 0.90 to 1.90, f(2) is 0.10 to 0.40, f(3) is 0 to 0.40, and a total of CaO, SiO.sub.2, F, S, Li.sub.2O, Na.sub.2O, and K.sub.2O is 90 to 100 mass %.
Claims
1. A mold flux for continuous casting which has a base material composition containing: 25 to 60 mass % of CaO; 15 to 45 mass % of SiO.sub.2; 5 to 25 mass % of F; 0.5 to 1.00 mass % of S; and 0 to 20 mass % of a total of Li.sub.2O, Na.sub.2O, and K.sub.2O, wherein in the base material composition, f(1) represented by Formula 1 is 0.90 to 1.90, f(2) represented by Formula 2 is 0.10 to 0.40, f(3) represented by Formula 3 is 0 to 0.40, and a total of CaO, SiO.sub.2, F, S, Li.sub.2O, Na.sub.2O, and K.sub.2O is 90 to 100 mass %,
f(1)=(CaO).sub.h/(SiO.sub.2).sub.h (Formula 1)
f(2)=(CaF.sub.2).sub.h{(CaO).sub.h+(SiO.sub.2).sub.h+(CaF.sub.2).sub.h} (Formula 2)
f(3)=(MF).sub.h/{(CaO).sub.h+(SiO.sub.2).sub.h+(MF).sub.h}tm (Formula 3) where (CaO).sub.h, (SiO.sub.2).sub.h, (CaF.sub.2).sub.h, and (MF).sub.h in Formulae 1 to 3 are calculated as follows,
(CaO).sub.h=W.sub.CaO-(CaF.sub.2).sub.h×0.718 (Formula 4)
(SiO.sub.2).sub.h=W.sub.SiO2 (Formula 5)
(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 (Formula 6)
(MF).sub.h=W.sub.Li2O×1.74+W.sub.Na2O×1.35+W.sub.K2O×1.23 (Formula 7) where W.sub.i represents a percentage by mass of each component i.
2. The mold flux for continuous casting according to claim 1, wherein the amount of S is 0.6 to 1.0 mass % in the base material composition.
3. The mold flux for continuous casting according to claim 1, wherein the base material composition further contains 0 to 4.0 mass % of Al.sub.2O.sub.3.
4. The mold flux for continuous casting according to claim 1, wherein the base material composition has a solidification point of 1150° C. to 1400° C.
5. The mold flux for continuous casting according to claim 1, wherein the base material composition has a viscosity of 2 poise or less at 1300° C.
6. The mold flux for continuous casting according to claim 1, wherein the base material composition has a basicity of 1.2 to 2.1.
7. The mold flux for continuous casting according to claim 1, wherein a total of CaO, SiO.sub.2, F, S, Li.sub.2O, Na.sub.2O, and K.sub.2O is 90 to 98 mass % in the base material composition.
8. The mold flux for continuous casting according to claim 1, wherein 0 to 10 parts by mass of C is contained with respect to 100 parts by mass of the base material composition.
9. A continuous casting method comprising: casting steel having a steel composition containing 0.10 to 3.00 mass % of Al, using the mold flux for continuous casting according to claim 1.
10. The continuous casting method according to claim 9, wherein the steel composition further contains 0.06 to 0.20 mass % of C.
11. The continuous casting method according to claim 9, wherein the steel composition contains 0.10 to 3.00 mass % of Al, 0 to 0.20 mass % of C, 0 to 1.0 mass % of Si, 0 to 3.0 mass % of Mn, 0 to 0.030 mass % of P, 0 to 0.010 mass % of S, 0 to 0.30 mass % of each of Cu, Ni, V, Nb, Ti, Cr, Mo, W, and Zr, 0 to 0.030 mass % of each of Ca, Mg, REM, and B, and the remainder of Fe with impurities.
12. The continuous casting method according to claim 11, wherein the amount of C is 0.06 to 0.20 mass % in the steel composition.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1)
(2)
EMBODIMENTS OF THE INVENTION
(3) Hereinafter, a mold flux according to an embodiment of the invention and a continuous casting method according to an embodiment of the invention will be shown.
(4) In these embodiments, steel containing 0.10 mass % or greater of Al is defined as Al-containing steel. The steel may contain up to 3.00 mass % or less of Al in order to increase the strength and the corrosion resistance of the steel. Ca.sub.4Si.sub.2O.sub.7F.sub.2 is crystallized in a film even in a case where the steel contains 3.00 mass % of Al.
(5) A base material of a mold flux according to this embodiment contains 25 to 60 mass % of CaO, 15 to 45 mass % of SiO.sub.2, 0 to 20 mass % of Na.sub.2O, Li.sub.2O, and K.sub.2O (group of three kinds of alkali metal oxides) in total, 5 to 25 mass % of F, and 0.20 to 1.00 mass % of S. The mold flux according to this embodiment includes the base material, and if necessary, may include a carbon material such as a coke powder or a fine-particle carbon powder in order to adjust a melting rate. The amount of the carbon material is defined by a concentration in outer percentage relative to 100 mass % of the base material (total amount of components in the base material), and 0 to 10 mass % (10 parts by mass) with respect to 100 mass % of the base material (100 parts by mass). The mold flux according to this embodiment is defined by a composition (initial composition) before adding to a mold. In addition, the composition of the mold flux according to this embodiment is defined by a conventional method in this field as shown in Example 1 to be described later.
(6) Furthermore, in the composition of the base material of the mold flux according to this embodiment, f(1) represented by Formula 1 is 0.90 to 1.90, f(2) represented by Formula 2 is 0.10 to 0.40, and f(3) represented by Formula 3 is 0 to 0.40. The ranges of f(1), f(2), and f(3) are as shown in
f(1)=(CaO).sub.h/(SiO.sub.2).sub.h (Formula 1)
f(2)=(CaF.sub.2).sub.h/{(CaO).sub.h+(SiO.sub.2).sub.h+(CaF.sub.2).sub.h} (Formula 2)
f(3)=(MF).sub.h/{(CaO).sub.h+(SiO.sub.2).sub.h+(MF).sub.h} (Formula 3)
(7) Here, in Formulae 1 to 3, (CaO).sub.h is calculated by Formula 4, (SiO.sub.2).sub.h is calculated by Formula 5, (CaF.sub.2).sub.h is calculated by Formula 6, and (MF).sub.h is calculated by Formula 7.
(CaO).sub.h=W.sub.CaO−(CaF.sub.2).sub.h×0.718 (Formula 4)
(SiO.sub.2).sub.h=W.sub.SiO2 (Formula 5)
(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 (Formula 6)
(MF).sub.h=W.sub.Li2O×1.74+W.sub.Na2O×1.35+W.sub.K2O×1.23 (Formula 7)
(8) In Formulae 4 to 7, W.sub.i represents a percentage by mass of each component i with respect to the total mass (total amount of components in the base material) of the base material. That is, W.sub.CaO represents a percentage by mass of CaO with respect to the total mass of the base material, W.sub.SiO2 represents a percentage by mass of SiO.sub.2 with respect to the total mass of the base material, W.sub.F represents a percentage by mass of F with respect to the total mass of the base material, W.sub.Li2O represents a percentage by mass of Li.sub.2O with respect to the total mass of the base material, W.sub.Na2O represents a percentage by mass of Na.sub.2O with respect to the total mass of the base material, and W.sub.K2O represents a percentage by mass of K.sub.2O with respect to the total mass of the base material. f(1), f(2), f(3), (CaO).sub.h, (SiO.sub.2).sub.h, (CaF.sub.2).sub.h, and (MF).sub.h may be expressed as f1, f2, f3, h.sub.CaO, h.sub.SiO2, h.sub.CaF2, and h.sub.MF, respectively. As understood from Formulae 2 and 3 and
(9) f(1) is different from a usual basicity defined by a mass ratio of CaO with respect to SiO.sub.2. Since the mass of CaO is reduced with an increase in the mass of CaF.sub.2, a numerator (mass of CaO) in the usual basicity is replaced with (CaO).sub.h in f(1). As above, f(1) is a modified basicity, and is important for promoting the crystallization of Ca.sub.4Si.sub.2O.sub.7F.sub.2. Therefore, the basicity is not required to be limited. The basicity (CaO/SiO.sub.2) of the base material of the mold flux according to this embodiment may be 1.2 to 2.1.
(10) Therefore, as shown in Formula 8, the range of f(1) is 0.90 to 1.90. The range of f(1) is determined in consideration of a reduction of SiO.sub.2 during continuous casting due to an oxidation-reduction reaction with Al (└Al┘≥0.10 mass %) in molten steel. A preferable lower limit of f(1) is 0.95, 1.00, or 1.05. A preferable upper limit of f(1) is 1.85, 1.80, or 1.75. For example, a desirable range of f(1) is 0.95 to 1.90, 1.00 to 1.90, 1.05 to 1.90, 0.90 to 1.85, 0.95 to 1.85, 1.00 to 1.85, 1.05 to 1.85, 0.90 to 1.80, 0.95 to 1.80, 1.00 to 1.80, or 1.05 to 1.80.
0.90≤f(1)≤1.90 (Formula 8)
(11) In a case where f(1) is less than 0.90 or greater than 1.90, a required amount of a crystal phase of Ca.sub.4Si.sub.2O.sub.7F.sub.2 cannot be obtained, and thus the cooling in the mold becomes unstable, and the temperature of a copper plate of the mold largely fluctuates.
(12) f(2) is a mass ratio of (CaF.sub.2).sub.h with respect to the total amount of components constituting a (CaO).sub.h—(SiO.sub.2).sub.h—(CaF.sub.2).sub.h system phase diagram. f(2) is also required to be adjusted within an appropriate range in order to promote the crystallization of Ca.sub.4Si.sub.2O.sub.7F.sub.2. Therefore, as shown in Formula 9, the range of f(2) is 0.10 to 0.40. In a case where f(2) is less than 0.10 or greater than 0.40, a sufficient amount of a crystal phase of Ca.sub.4Si.sub.2O.sub.7F.sub.2 cannot be obtained. A preferable lower limit of f(2) is 0.11. A preferable upper limit of f(2) is 0.35, 0.30, 0.25, or 0.20. For example, a desirable range of f(2) is 0.11 to 0.40, 0.11 to 0.35, 0.11 to 0.30, 0.11 to 0.25, 0.11 to 0.20, 0.10 to 0.35, 0.11 to 0.30, 0.11 to 0.25, or 0.11 to 0.20.
0.10≤f(2)≤0.40 (Formula 9)
(13) f(3) represents a ratio of a component acting as a solvent which dissolves Ca.sub.4Si.sub.2O.sub.7F.sub.2, that is, (MF).sub.h with respect to the total amount of components constituting a (CaO).sub.h—(SiO.sub.2).sub.h-(MF).sub.h system phase diagram. Accordingly, f(3) is also required to be adjusted within an appropriate range in order to promote the crystallization of Ca.sub.4Si.sub.2O.sub.7F.sub.2. Therefore, as shown in Formula 10, the range of f(3) is 0 to 0.40. The range of f(3) is determined in consideration of a reduction of SiO.sub.2 due to an oxidation-reduction reaction with Al (┌Al┐≥0.10 mass %) in molten steel. In a case where f(3) is greater than 0.40, a sufficient amount of a crystal phase of Ca.sub.4Si.sub.2O.sub.7F.sub.2 cannot be obtained. A preferable upper limit of f(3) is 0.35, 0.30, 0.25, or 0.20. For example, a desirable range of f(3) is 0 to 0.35, 0 to 0.30, 0 to 0.25, or 0 to 0.20.
0≤f(3)≤0.40 (Formula 10)
(14) As described above, the base material of the mold flux according to this embodiment is required to contain 0.20 to 1.00 mass % of S. When the amount of S is 0.20 to 1.00 mass %, a temporal change in the composition of the molten layer is small even when slabs are manufactured from molten steel of Al-containing steel by continuous casting. Therefore, the inflow velocity of the molten slag along an inner wall of the mold becomes uniform. In addition, even in a case where Al in the molten steel reacts with the molten slag in the film, and thus the film composition changes, the rate of the crystallization of Ca.sub.4Si.sub.2O.sub.7F.sub.2 in the film can be maintained. As a result, the thickness of a solidified shell is likely to be uniformized, and the fluctuation of the temperature of a copper plate of the mold is reduced. A preferable lower limit of the amount of S is 0.30, 0.50, 0.60, or 0.65 mass %. A preferable upper limit of the amount of S is 0.95 mass %.
(15) When the amount of S is less than 0.20 mass %, the molten layer has an unstable composition, the temperature of a copper plate of the mold largely fluctuates, and cracks occur on a slab surface. When the amount of S is greater than 1.00 mass %, a negative effect of S transferred in the molten steel on the interfacial tension of the molten steel and the toughness of the steel offsets a preferable effect of S in the molten slag on the fluctuation of the temperature of a copper plate of the mold, and thus cracks occur on a slab surface.
(16) In order to generate a sufficient amount of Ca.sub.4Si.sub.2O.sub.7F.sub.2 (cuspidine: 3CaO/2SiO.sub.2/CaF.sub.2) in the film at a sufficient speed, a predetermined amount of Ca, a predetermined amount of Si, and a predetermined amount of F are required in the film. Therefore, as described above, the base material of the mold flux according to this embodiment contains 25 to 60 mass % of CaO, 15 to 45 mass % of SiO.sub.2, and 5 to 25 mass % of F as essential components for generating Ca.sub.4Si.sub.2O.sub.7F.sub.2 (cuspidine: 3CaO/2SiO.sub.2/CaF.sub.2) in the film. When the amounts of the essential components are not sufficient, a sufficient amount of a crystal phase of Ca.sub.4Si.sub.2O.sub.7F.sub.2 cannot be obtained in the film. A preferable lower limit of the amount of CaO is 30 or 35 mass %. A preferable upper limit of the amount of CaO is 55 or 50 mass %. A preferable lower limit of the amount of SiO.sub.2 is 20 or 25 mass %. A preferable upper limit of the amount of SiO.sub.2 is 40 or 35 mass %. A preferable lower limit of the amount of F is 8 or 10 mass %. A preferable upper limit of the amount of F is 20 or 15 mass %.
(17) An alkali metal has high affinity to halogen such as F. When the mold flux contains alkali metal oxides such as Na.sub.2O, Li.sub.2O, and K.sub.2O in addition to CaO, SiO.sub.2, and F (that is, fluorine in fluoride), alkali metal ions in the alkali metal oxides are bonded to fluorine ions in CaF2 as in Formulae 11 to 13 in the molten slag generated from the mold flux.
CaF.sub.2+Li.sub.2O.fwdarw.CaO+2LiF (Formula 11)
CaF.sub.2+Na.sub.2O.fwdarw.CaO+2NaF (Formula 12)
CaF.sub.2+K.sub.2O.fwdarw.CaO+2KF (Formula 13)
(18) Therefore, Li.sub.2O, Na.sub.2O, and K.sub.2O in the mold flux are regarded as LiF, NaF, and KF, respectively. In addition, CaF.sub.2 of which anions are exchanged with that of the alkali metal oxides is regarded as CaO. As a result, Formulae 4, 6, and 7 are obtained. Li, Na, K, Rb, Cs, and Fr are alkali metals. However, in a case where an alkali metal is added to the mold flux, one or more of Li, Na, and K is preferable as the alkali metal. Li.sub.2O, Na.sub.2O, and K.sub.2O are more easily available than other alkali metal oxides (Rb.sub.2O, Cs.sub.2O, and Fr.sub.2O). Since the addition of Rb.sub.2O, Cs.sub.2O, and Fr.sub.2O to the mold flux is industrially very disadvantageous, these are regarded as other components to be described later.
(19) The base material of the mold flux according to this embodiment may contain at least one selected from the group consisting of Na.sub.2O, Li.sub.2O, and K.sub.2O as an optional component for adjusting a solidification point. However, in a case where the amount of the alkali metal oxides is too large, the amount of the above-described essential components is not sufficient. Therefore, when the total amount of the alkali metal oxides is greater than 20 mass %, a sufficient amount of a crystal phase of Ca.sub.4Si.sub.2O.sub.7F.sub.2 cannot be obtained. Accordingly, the total amount of Na.sub.2O, Li.sub.2O, and K.sub.2O is 0 to 20 mass %. A preferable upper limit of the total amount is 18, 15, 12, 10, or 8 mass %.
(20) CaO, SiO.sub.2, F, S, Li.sub.2O, Na.sub.2O, and K.sub.2O are base elements of the base material of the mold flux according to this embodiment. In order to generate a sufficient amount of a crystal phase of Ca.sub.4Si.sub.2O.sub.7F.sub.2, the total amount of the base elements (CaO, SiO.sub.2, F, S, Li.sub.2O, Na.sub.2O, and K.sub.2O) is required to be 90 to 100 mass %. In a case where the total amount of the base elements is less than 90%, a required amount of a crystal phase of Ca.sub.4Si.sub.2O.sub.7F.sub.2 cannot be obtained. It is not necessary to limit the upper limit of the total amount of the base elements. In a case where the base material of the mold flux contains other components to be described later, the total amount of the base elements may be 98 mass % or less.
(21) The base material of the mold flux according to this embodiment may contain other components other than the base elements. For example, the base material of the mold flux according to this embodiment may contain 0.1 to 10.0 mass % of Al.sub.2O.sub.3, 0.1 to 10.0 mass % of MgO, and 0.1 to 4.0 mass % of MnO. However, since the total amount of the base elements is required to be 90 mass % or greater, the total amount of other components is required to be 10 mass % or less. That is, the total amount of other components is 0 to 10 mass %. In addition, the amount of Al.sub.2O.sub.3 is 0 to 10.0 mass %, and the amount of MgO is 0 to 10.0 mass %. The amount of Al.sub.2O.sub.3 is more preferably 0 to 4.0 mass %, the amount of MgO is more preferably 0 to 4.0 mass %, and the amount of MnO is more preferably 0 to 4.0 mass %.
(22) The solidification point of the base material of the mold flux according to this embodiment is desirably 1150° C. to 1400° C. In a case where the solidification point is within this temperature range, the crystallization of Ca.sub.4Si.sub.2O.sub.7F.sub.2 can be further promoted. The solidification point of the flux is measured by a rotation-type or vibration piece-type viscosity measuring device. A more preferable lower limit of the solidification point is 1200° C., 1240° C., or 1250° C. A more preferable upper limit of the solidification point is 1350° C. or 1300° C.
(23) The viscosity of the base material of the mold flux according to this embodiment is desirably 2 poise or less (0.2 Pa.Math.s or less) at 1300° C. 2 poise or less of viscosity is effective for gradual cooling since the crystallization rate can be further increased. The mold flux desirably has a low viscosity since the composition of the molten layer changes due to the reaction in the mold, and thus the viscosity of the molten slag during casting is higher than the viscosity of the molten slag at initial phase of casting. The viscosity of the flux is measured by a rotation-type or vibration piece-type viscosity measuring device as in the case of the solidification point. A more preferable upper limit of the viscosity of the flux is 1 poise (0.1 Pa.Math.s).
(24) In a continuous casting method according to an embodiment of the invention, steel (molten steel) contains 0.10 to 3.00 mass % of Al. In order to further increase the strength of completed products, the steel may contain 0.06 to 0.20 mass % of C. For example, a high strength steel sheet as a completed product preferably has a tensile strength of 780 MPa or greater. Therefore, the steel composition in the steel may be adjusted such that the tensile strength after hot rolling and cold rolling is 780 MPa or greater.
(25) As described above, in the continuous casting method according to this embodiment, steel contains Al as an essential element. In addition, in the continuous casting method according to this embodiment, the steel may contain at least one selected from the group consisting of C, Si, Mn, P, S, Cu, Ni, V, Nb, Ti, Cr, Mo, W, Zr, Ca, Mg, REM, and B as an optional element. The remainder is Fe and impurities. For example, steel may contain 0.10 to 3.00 mass % of Al, 0 to 0.20 mass % of C, 0 to 1.0 mass % of Si, 0 to 3.0 mass % of Mn, 0 to 0.03 mass % of P, 0 to 0.01 mass % of S, 0 to 0.30 mass % of Cu, 0 to 0.30 mass % of Ni, 0 to 0.30 mass % of V, 0 to 0.30 mass % of Nb, 0 to 0.30 mass % of Ti, 0 to 0.30 mass % of Cr, 0 to 0.30 mass % of Mo, 0 to 0.30 mass % of W, 0 to 0.30 mass % of Zr, 0 to 0.030 mass % of Ca, 0 to 0.030 mass % of Mg, 0 to 0.030 mass % of REM, 0 to 0.030 mass % of B, and the remainder of Fe with impurities.
(26) For example, the amount of Si may be 0.02 to 1.0 mass %, and the amount of Mn may be 0.5 to 3.0 mass %. In order to improve the strength and workability of a high strength steel sheet as a completed product, at least one selected from the group consisting of Cu, Ni, V, Nb, Ti, Cr, Mo, W, and Zr may be contained in an amount of 0.30 mass % or less, respectively, in steel. At least one selected from the group consisting of Ca, Mg, REM, and B may be further contained in an amount of 0.030 mass % or less, respectively, in the steel. It is not necessary to limit the lower limit of the amount of each optional element. For example, the amount of each optional element may be equal to or greater than 0%, or greater than 0%.
(27) In the continuous casting method according to this embodiment, steel having the above-described steel composition is cast using the mold flux according to the embodiment. In the continuous casting method according to this embodiment, even if the film composition changes with an oxidation reaction caused by Al in molten steel of Al-containing steel in a mold in which molten slag is formed from the mold flux, it is possible to maintain the crystallization of Ca.sub.4Si.sub.2O.sub.7F.sub.2 while permitting the crystallization of Ca.sub.12Al.sub.14F.sub.2O.sub.32 in the film, and to gradually cool a front end section of a solidified shell in a comprehensive manner. In addition, in a case where the amount of C is 0.06 to 0.20 mass %, the steel may be hypo-peritectic steel, and thus surface cracking is likely to occur in slabs in conventional methods. In this case, surface cracking can be prevented using gradual cooling by the crystallization in the film.
(28) Furthermore, during continuous casting, SiO.sub.2 is reduced by Al ([Al]≥0.10 mass %) in molten steel, and the amount thereof is thus reduced. Accordingly, a mold flux in which f(1) is low may be selected according to the concentration of Al in the molten steel. In addition, the composition of the molten layer or film during the continuous casting may be measured or simulated, and the relationship between f(1) calculated from the composition of the molten layer or film and f(1) calculated from the composition of the mold flux may be determined. Based on this relationship, an appropriate mold flux can be selected. Similarly, in a case where SiO.sub.2 is reduced by Al in the molten steel, f(3) calculated from the composition of the molten layer and film is increased. An increase of f(3) has an effect on the crystallization of Ca.sub.4Si.sub.2O.sub.7F.sub.2. Accordingly, an appropriate mold flux may be selected such that, for example, the film composition is 0.40 or less.
EXAMPLE 1
(29) 780 tons of molten steel having a composition shown in Table 1 was cast using a vertical bending-type continuous casting machine having two strands. In both of the strands, slabs having a width of 1500 mm, a thickness of 250 mm, and a length of 7000 mm were obtained. The casting speed was set to 1.5 m/min. In continuous casting, mold fluxes shown in Tables 2 to 5 were differently used for each strand.
(30) TABLE-US-00001 TABLE 1 STEEL COMPOSITION (mass %) C Si Mn P S Al 0.12 0.15 2.30 0.012 0.003 0.8
(31) TABLE-US-00002 TABLE 2 MOLD COMPOSITION (mass %) FLUX SiO.sub.2 CaO Al.sub.2O.sub.3 MgO Na.sub.2O MnO F S C 1 32.4 45.4 2.0 0.9 6.5 1.2 11.5 0.21 6.0 INVENTION (30.5) (42.6) (1.9) (0.8) (6.1) (1.1) (10.8) (0.20) (6.0) EXAMPLE 2 28.3 48.0 2.1 0.9 6.4 1.3 12.7 0.43 6.0 INVENTION (26.6) (45.1) (2.0) (0.8) (6.0) (1.2) (11.9) (0.40) (6.0) EXAMPLE 3 31.9 44.7 1.9 0.9 6.2 1.6 12.2 0.64 6.0 INVENTION (30.0) (42.0) (1.8) (0.8) (5.8) (1.5) (11.5) (0.60) (6.0) EXAMPLE 4 28.3 48.0 2.2 0.9 5.3 1.6 12.9 0.85 6.0 INVENTION (26.6) (45.2) (2.1) (0.8) (5.0) (1.5) (12.1) (0.80) (6.0) EXAMPLE 5 35.6 42.8 2.3 0.9 4.8 1.4 11.7 0.53 6.0 INVENTION (33.5) (40.2) (2.2) (0.8) (4.5) (1.3) (11.0) (0.50) (6.0) EXAMPLE 6 24.1 50.5 4.2 0.9 6.9 0.5 12.5 0.43 6.0 INVENTION (22.6) (47.5) (3.9) (0.8) (6.5) (0.5) (11.7) (0.40) (6.0) EXAMPLE 7 24.1 50.5 4.2 0.9 6.9 0.5 12.5 0.53 6.0 INVENTION (22.6) (47.5) (3.9) (0.8) (6.5) (0.5) (11.7) (0.50) (6.0) EXAMPLE
(32) TABLE-US-00003 TABLE 3 MOLD COMPOSITION (mass %) FLUX SiO.sub.2 CaO Al.sub.2O.sub.3 MgO Na.sub.2O MnO F S C 8 32.2 45.1 2.0 0.9 6.4 1.5 11.9 0.11 6.0 COMPARATIVE (30.3) (42.4) (1.9) (0.8) (6.0) (1.4) (11.2) (0.10) (6.0) EXAMPLE 9 28.7 48.8 2.3 0.9 5.9 1.4 12.0 0.05 6.0 COMPARATIVE (27.0) (45.9) (2.2) (0.8) (5.5) (1.3) (11.3) (0.05) (6.0) EXAMPLE 10 28.2 47.9 2.0 0.9 6.5 1.2 12.1 1.28 6.0 COMPARATIVE (26.5) (45.0) (1.9) (0.8) (6.1) (1.1) (11.4) (1.20) (6.0) EXAMPLE 11 37.4 37.4 2.7 0.9 6.9 2.1 12.2 0.43 6.0 COMPARATIVE (35.2) (35.2) (2.5) (0.8) (6.5) (2.0) (11.5) (0.40) (6.0) EXAMPLE 12 23.0 52.8 1.6 0.9 7.8 1.0 12.7 0.43 6.0 COMPARATIVE (21.6) (49.6) (1.5) (0.8) (7.3) (0.9) (11.9) (0.40) (6.0) EXAMPLE 13 29.5 50.1 2.6 0.9 6.9 1.2 8.5 0.43 6.0 COMPARATIVE (27.7) (47.1) (2.4) (0.8) (6.5) (1.1) (8.0) (0.40) (6.0) EXAMPLE
(33) TABLE-US-00004 TABLE 4 SOLIDIFICATION MOLD BASICITY POINT VISCOSITY FLUX f(1) f(2) f(3) (−) (° C.) (poise) 1 1.20 0.11 0.11 1.40 1254 0.8 INVENTION EXAMPLE 2 1.40 0.15 0.11 1.70 1256 0.7 INVENTION EXAMPLE 3 1.15 0.14 0.11 1.40 1252 0.9 INVENTION EXAMPLE 4 1.34 0.18 0.10 1.70 1245 0.5 INVENTION EXAMPLE 5 0.94 0.16 0.09 1.20 1247 0.9 INVENTION EXAMPLE 6 1.80 0.13 0.12 2.10 1242 0.5 INVENTION EXAMPLE 7 1.80 0.13 0.12 2.10 1242 0.5 INVENTION EXAMPLE
(34) TABLE-US-00005 TABLE 5 SOLIDIFICATION MOLD BASICITY POINT VISCOSITY FLUX f(1) f(2) f(3) (−) (° C.) (poise) 8 1.18 0.13 0.11 1.40 1255 0.7 COMPARATIVE EXAMPLE 9 1.42 0.14 0.10 1.70 1249 0.6 COMPARATIVE EXAMPLE 10 1.44 0.13 0.11 1.70 1253 0.7 COMPARATIVE EXAMPLE 11 0.81 0.12 0.12 1.00 1150 1.2 COMPARATIVE EXAMPLE 12 2.03 0.11 0.13 2.30 1188 0.4 COMPARATIVE EXAMPLE 13 1.65 0.03 0.11 1.70 1176 0.7 COMPARATIVE EXAMPLE
(35) In order to measure the composition of molten steel, the molten steel was sampled by inserting an analysis sampler into the molten steel in a mold. After solidification of the molten steel, emission spectrometric analysis was performed to find the composition of the molten steel. The obtained molten steel composition is shown in Table 1. Components other than the components shown in Table 1 were iron and impurities.
(36) The composition of a mold flux was determined by a conventional method. That is, the amount (mass concentration) of an element, which could generally exist as a cation usually measured, was obtained by fluorescent X-ray analysis (JSX-3200 manufactured by JEOL Ltd.). The obtained amount of each element was converted into an amount of a general oxide corresponding to each element. The amount of S and the amount of C were determined by a combustion method, and the amount of F was determined by a conventional method. In the upper fields in Tables 2 and 3, the amounts of the respective components (respective oxides, F, S, and C) determined based on the total amount (total amount of the components excluding C) of a base material set to 100% are shown. The base material means components in a mold flux which form molten slag in a mold. Accordingly, C, which has little effect on the composition of the molten slag, is excluded from the base material. In the lower fields in Tables 2 and 3, the amounts of the respective components determined based on the total amount of all the components set to 100% are also shown in brackets for reference. In Tables 3 and 5, the lower lines show that the conditions according to the invention are not satisfied. In Tables 4 and 5, the basicity is CaO/SiO.sub.2, and a ratio of the amount of CaO to the amount of SiO.sub.2.
(37) A mold flux melted in a graphite crucible at 1400° C. was cooled at a rate of 2° C./min, and the solidification point and the viscosity of the mold flux were measured by a vibration piece-type viscosity measuring device in the course of cooling. The viscosity was determined at 1300° C. The temperature at which the viscosity started to rapidly increase during the solidification of the melted mold flux was regarded as the solidification point. A measurement device based on the specifications disclosed in Non-Patent Document 2 was used. The obtained viscosity and solidification point are shown in Tables 4 and 5.
(38) f(1), f(2), and f(3) shown in Tables 4 and 5 were calculated from the amounts of the components shown in Tables 2 and 3 (upper fields). The temperature of a copper plate of the mold was measured by a thermocouple thermometer. The thermocouple thermometer was positioned below a surface of the molten steel by 100 mm at a center of the width of an inner long-side surface of the mold. A temporal change in the measurement value of the thermocouple thermometer was monitored to determine an average value of the temperature of the copper plate and an amount of temperature fluctuation of the copper plate. In addition, the number of times of issuing prediction-warning of breakout was counted. Furthermore, whole surfaces of front and rear surfaces of a slab, having a width of 1500 mm and a length of 7000 mm, were visually examined. The number of cracks having a length of 10 mm or greater in a length direction of the slab was counted. A slab having 5 or more of cracks was defined as a slab having cracks, and the number of slabs having 5 or more of cracks was counted. The average temperature and the temperature range of a copper plate of the mold, the number of times of issuing prediction-warning of breakout, the average casting speed, and the number of slabs having cracks are shown in Tables 6 and 7.
(39) TABLE-US-00006 TABLE 6 NUMBER OF TIMES OF NUMBER TEMPERATURE OF ISSUING AVERAGE OF SLABS COPPER PLATE OF MOLD PREDICTION- CASTING HAVING MOLD AVERAGE AMOUNT OF WARNING OF SPEED CRACKS FLUX VALUE FLUCTUATION BREAKOUT (m/min) (number) 1 135° C. 20° C. 0 1.35 0 INVENTION EXAMPLE 2 138° C. 15° C. 0 1.33 0 INVENTION EXAMPLE 3 137° C. 15° C. 0 1.34 0 INVENTION EXAMPLE 4 141° C. 10° C. 0 1.31 0 INVENTION EXAMPLE 5 144° C. 15° C. 0 1.30 0 INVENTION EXAMPLE 6 136° C. 20° C. 0 1.30 0 INVENTION EXAMPLE 7 136° C. 15° C. 0 1.35 0 INVENTION EXAMPLE
(40) TABLE-US-00007 TABLE 7 NUMBER OF NUMBER TIMES OF OF TEMPERATURE OF ISSUING AVERAGE SLABS COPPER PLATE OF MOLD PREDICTION- CASTING HAVING MOLD AVERAGE AMOUNT OF WARNING OF SPEED CRACKS FLUX VALUE FLUCTUATION BREAKOUT (m/min) (number) 8 143° C. 30° C. 3 0.85 3 COMPARATIVE EXAMPLE 9 141° C. 50° C. 5 0.79 5 COMPARATIVE EXAMPLE 10 145° C. 10° C. 0 0.80 18 COMPARATIVE EXAMPLE 11 151° C. 40° C. 0 1.10 3 COMPARATIVE EXAMPLE 12 140° C. 50° C. 7 0.75 7 COMPARATIVE EXAMPLE 13 148° C. 25° C. 0 1.10 8 COMPARATIVE EXAMPLE
(41) As shown in Table 6, in the invention examples, the amount of temperature fluctuation of a copper plate of the mold was 20° C. or lower, the prediction-warning of breakout was not issued, and the average casting speed was 1.30 m/min or more. In the invention examples, slabs having cracks were not provided.
(42) As shown in Table 7 (comparative examples), the amount of S in the mold fluxes 8 and 9 was less than 0.20%, the amount of temperature fluctuation of a copper plate of the mold was greater than 20° C., and the cooling in the mold became unstable. Accordingly, the prediction-warning of breakout was issued, and the casting speed was necessarily reduced. As a result, the casting efficiency was reduced with the average casting speed. In addition, cracking of a slab surface also occurred with a change of the casting speed. In the mold flux 10, the amount of S was greater than 1.00%, and a large amount of S was transferred to the molten steel from the mold flux (molten slag). Thus, cracking occurred on a slab surface. In the mold fluxes 8 to 10, the average casting speed was slower than 1.0 m/min.
(43) In addition, as shown in Table 7 (comparative examples), in the mold flux 11, f(1) was less than 0.90, and thus the amount of temperature fluctuation of a copper plate of the mold was greater than 20° C. In the mold flux 12, f(1) was greater than 1.90, and thus the amount of temperature fluctuation of a copper plate of the mold was greater than 20° C., and the cooling in the mold became unstable. Accordingly, the prediction-warning of breakout was issued, and the casting speed was necessarily reduced. As a result, the casting efficiency was reduced with the average casting speed. In the mold flux 13, f(2) was less than 0.10, and thus the amount of temperature fluctuation of a copper plate of the mold was greater than 20° C. In the mold fluxes 11 to 13, cracking occurred on a slab surface.
EXAMPLE 2
(44) Using the mold flux 1 of Example 1, 260 tons of 9 kinds of molten steel, each having a composition shown in Table 8, was continuously cast to obtain 9 kinds of slabs.
(45) As a continuous casting machine, a vertical bending-type continuous casting machine having two strands was used as in Example 1. In each casting, a total of 12 slabs having a width of 1500 mm, a thickness of 250 mm, and a length of 7000 mm were obtained in both the strands. The casting speed was set to 1.5 m/min.
(46) In order to measure the composition of molten steel, the molten steel was sampled by inserting an analysis sampler into the molten steel in a mold. After solidification of the molten steel, emission spectrometric analysis was performed to obtain the composition of the molten steel. The obtained molten steel composition is shown in Table 8. Components other than the components shown in Table 8 were iron and impurities.
(47) The results of the continuous casting are shown in Table 9. In all of the compositions, 260 tons of the molten steel was completely cast with a stable copper plate temperature of the mold and no prediction-warning of breakout. As a result, good slabs having no surface cracking and no dimples were obtained. Steel sheets were manufactured from the slabs by hot rolling and subsequent cold rolling.
(48) A sample was collected from the steel sheets subjected to cold rolling, and a tensile strength thereof was measured by a tension tester. The measured tensile strengths are shown in Table 9. As shown in Table 9, any steel sheet has a tensile strength of 780 MPa or greater.
(49) TABLE-US-00008 TABLE 8 STEEL COMPOSITION (mass %) STEEL C Si Mn P S Nb Ti Cr Mo B Al A 0.06 0.05 1.50 0.020 0.0030 0.02 0.10 — — — 0.30 B 0.06 0.05 2.00 0.020 0.0030 0.02 0.15 — — — 0.30 C 0.12 0.05 2.20 0.020 0.0030 0.01 — — 0.06 0.0008 0.70 D 0.09 0.60 1.60 0.020 0.0030 0.01 0.13 0.09 0.10 — 0.20 E 0.20 0.80 2.20 0.020 0.0030 — 0.05 — — — 0.30 F 0.13 0.05 2.55 0.020 0.0020 0.01 — 0.30 — — 0.62 G 0.14 0.10 2.50 0.020 0.0020 0.01 — 0.20 — 0.0015 0.38 H 0.15 0.50 2.45 0.020 0.0020 — 0.02 0.20 — 0.0010 0.30 I 0.13 0.50 2.80 0.020 0.0020 0.01 0.02 0.10 — 0.0015 0.21
(50) TABLE-US-00009 TABLE 9 AMOUNT OF NUMBER OF NUMBER TENSILE TEMPERATURE TIMES OF OF SLABS STRENGTH FLUCTUATION ISSUING HAVING OF COM- OF COPPER PREDICTION- CRACKS PLETED PLATE OF WARNING OF AND PRODUCT STEEL MOLD BREAKOUT DIMPLES (MPa) A 20° C. or less 0 0 810 B 20° C. or less 0 0 870 C 20° C. or less 0 0 980 D 20° C. or less 0 0 1010 E 20° C. or less 0 0 1020 F 20° C. or less 0 0 1065 G 20° C. or less 0 0 1190 H 20° C. or less 0 0 1210 I 20° C. or less 0 0 1320
(51) Using the mold flux 9, molten steel of steel C was cast. The results of the continuous casting are shown in Table 11. While 260 tons of the molten steel were completely cast, the prediction-warning of breakout was issued three times, and the casting speed was necessarily reduced to 0.3 m/min. Among the 12 slabs, 6 slabs had surface cracks and were required to be subjected to scarfing repair of 3 mm. In addition, using the mold flux 9, steel D was cast. The temperature fluctuation of a copper plate of the mold was greater than 40° C., and the cooling in the mold became unstable. The prediction-warning of breakout was issued one time. Among the 12 slabs, 3 slabs had dimples or cracks on surfaces thereof, and thus scarfing repair of 3 mm was required. The steel C and the steel D in Table 10 are the same as the steel C and the steel D in Table 8.
(52) TABLE-US-00010 TABLE 10 STEEL COMPOSITION (mass %) STEEL C Si Mn P S Nb Ti Cr Mo B Al C 0.12 0.05 2.20 0.020 0.0030 0.01 — — 0.06 0.0008 0.70 D 0.09 0.60 1.60 0.020 0.0030 0.01 0.13 0.09 0.10 — 0.20
(53) TABLE-US-00011 TABLE 11 AMOUNT OF NUMBER OF TIMES TEMPERATURE OF ISSUING FLUCTUATION PREDICTION- NUMBER OF SLABS OF COPPER WARNING OF HAVING CRACKS STEEL PLATE OF MOLD BREAKOUT AND DIMPLES REMARKS C 30° C. 3 6 (CRACKS) THE CASTING SPEED WAS REDUCED TO 0.3 m/min. SCARFING REPAIR OF 3 mm WAS REQUIRED. D 40° C. 1 3 (CRACKS SCARFING REPAIR OF OR DIMPLES) 3 mm WAS REQUIRED.
INDUSTRIAL APPLICABILITY
(54) A mold flux for continuous casting and a continuous casting method for Al-containing steel are provided to prevent surface cracking of slabs of Al-containing steel.