Mold powder and method for inhibiting cast-slab surface cracks and bear occurrence

Abstract

To provide a mold powder allowed to suppress bear occurrence while inhibiting cracks in a cast-slab surface.SOLUTION: A mold powder containing SiO2, CaO, F, Na2O and Li2O comprises a SiO2 content (mass%) / 31.4, a CaO content (mass%) / 58.6 and a F content (mass%) / 10.0, a minimum value thereof being 0.6 or greater, the Na2O content and the Li2O content being totally 6 mass% or higher, the mold powder having a solidification temperature of 1200°C or lower and having a crystallization temperature of 500°C or higher and 660°C or lower.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a mold powder and a method for suppressing the occurrence of cracks and bare spots on the surface of a slab.

Background Art

[0002] A mold powder is put into a mold for continuous casting, melts by the heat of molten steel to become molten slag, and flows into the gap between the solidified steel (solidification shell) and the mold, playing a role in lubrication and heat extraction control. The properties of the mold powder affect the degree of cracks on the surface of the obtained slab. For example, in Patent Document 1, in order to prevent cracks on the surface of a medium carbon steel slab, by using a mold powder having a specific composition to promote crystal growth in the solidified mold powder (slag film) and roughen the surface roughness of the slag film, the solidification shell and molten steel can be slowly cooled, and it is disclosed that it is effective in preventing cracks on the slab surface.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] As disclosed in Patent Document 1, increasing the crystallinity of the mold powder is one of the measures to prevent cracks on the slab surface. However, according to the study of the present inventors, simply increasing the crystallinity of the mold powder alone results in excessive generation of bare spots (slag bare spots) at positions near the molten steel surface on the inner wall of the mold.

[0005] Therefore, one aspect of the present invention aims to provide a mold powder that can suppress the occurrence of bare spots while suppressing cracks on the slab surface.

Means for Solving the Problems

[0006] In addition to making the cuspidine ratio (details will be described later), which is an index of the crystallinity of the mold powder, equal to or greater than a specific value, the inventors have found that by setting the total amount of Na2O and Li2O and the solidification temperature and crystallization temperature of the mold powder within specific ranges, it is possible to suppress the occurrence of bare while also suppressing cracks on the surface of the slab.

[0007] The present invention includes the following aspects. [1] A mold powder containing SiO2, CaO, F, Na2O, and Li2O, wherein the minimum value among the content of SiO2 (% by mass) / 31.4, the content of CaO (% by mass) / 58.6, and the content of F (% by mass) / 10.0 is 0.6 or more, the total of the content of Na2O and the content of Li2O is 6% by mass or more, the solidification temperature of the mold powder is 1200°C or lower, and the crystallization temperature of the mold powder is 500°C or higher and 660°C or lower. [2] The mold powder according to [1], wherein the difference between the solidification temperature and the crystallization temperature is 490°C or more. [3] The mold powder according to [1] or [2], which is used for continuous casting of medium carbon steel. [4] A method for suppressing the occurrence of cracks on the surface of the slab and bare in the mold during continuous casting of steel, wherein as the mold powder, a mold powder containing SiO2, CaO, F, Na2O, and Li2O, wherein the minimum value among the content of SiO2 (% by mass) / 31.4, the content of CaO (% by mass) / 58.6, and the content of F (% by mass) / 10.0 is 0.6 or more, the total of the content of Na2O and the content of Li2O is 6% by mass or more, the solidification temperature of the mold powder is 1200°C or lower, and the crystallization temperature of the mold powder is 500°C or higher and 660°C or lower, is used to suppress the occurrence of cracks on the surface of the slab and bare. [5] The method according to [4], wherein the difference between the solidification temperature and the crystallization temperature is 490°C or more. [6] The method according to [4] or [5], wherein the steel is medium carbon steel.

Advantages of the Invention

[0008] According to one aspect of the present invention, it is possible to provide a mold powder that can suppress the generation of bare while suppressing cracks on the surface of the slab.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0010] Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

[0011] One embodiment of the present invention is a mold powder containing SiO2, CaO, F, Na2O, and Li2O.

[0012] In the mold powder of this embodiment, the cuspidine ratio is 0.6 or more. The cuspidine ratio may be 0.65 or more, 0.66 or more, 0.67 or more, 0.68 or more, 0.69 or more, or 0.70 or more, and may be 0.85 or less, 0.80 or less, 0.75 or less, or 0.73 or less.

[0013] The cuspidine ratio is (a) Content of SiO2 (mass%) / 31.4 (b) Content of CaO (mass%) / 58.6, and (c) Content of F (mass%) / 10.0 It is defined as the minimum value among them. The cuspidine ratio is an index indicating the proportion of cuspidine (3CaO·2SiO2·CaF2) as a crystal phase in the crystals that precipitate when the mold powder is used in the mold. The higher the cuspidine ratio, the greater the proportion occupied by cuspidine. 31.4 in (a) above, 58.6 in (b) above, and 10.0 in (c) above are the ratios (mass %) of SiO2, CaO, and F, respectively, when assuming that all the crystals are cuspidine.

[0014] The content of SiO2 (mass %) in (a) above, the content of CaO (mass %) in (b) above, and the content of F (mass %) in (c) above are the content of SiO2 (mass %), the content of CaO (mass %), and the content of F in the mold powder, respectively. In this specification, the content of these components and other components described later in the mold powder means the content (mass %) based on the total mass of the slag components. The content of each component in the mold powder (content based on the total mass of the slag components) is determined by the following procedure. First, weigh about 5 g of the mold powder and perform decarburization treatment at 700 °C for 5 minutes. Then, perform melting treatment on the mold powder at 1400 °C for 3 minutes and conduct component analysis of the treated sample. And the value obtained by converting each component value (multiplying by a coefficient at the same ratio) so that the sum of each component value becomes 100% is taken as the content based on the total mass of the slag components.

[0015] The content of SiO2 in the mold powder may be 20 mass % or more, 25 mass % or more, or 30 mass % or more, and may be 50 mass % or less, 45 mass % or less, or 40 mass % or less.

[0016] The content of CaO in the mold powder may be 25 mass % or more, 30 mass % or more, or 35 mass % or more, and may be 55 mass % or less, 50 mass % or less, or 45 mass % or less.

[0017] The mass ratio of the content of CaO to the content of SiO2 (CaO / SiO2) may be 1 or more, 1.1 or more, or 1.2 or more, and may be 1.6 or less, 1.5 or less, or 1.4 or less.

[0018] The content of F in the mold powder may be 6% by mass or more, 7% by mass or more, 8% by mass or more, or 9% by mass or more, and may be 14% by mass or less, 13% by mass or less, 12% by mass or less, 11% by mass or less, or 10% by mass or less.

[0019] In the mold powder of the present embodiment, the total of the content of Na2O and the content of Li2O is 6% by mass or more. The total of the content of Na2O and the content of Li2O may be 7% by mass or more, 8% by mass or more, or 9% by mass or more, and may be 15% by mass or less, 14% by mass or less, or 13% by mass or less.

[0020] The content of Na2O in the mold powder may be 1% by mass or more, 3% by mass or more, or 5% by mass or more, and may be 14% by mass or less, 12% by mass or less, or 10% by mass or less.

[0021] The content of Li2O in the mold powder may be 1% by mass or more, 3% by mass or more, or 5% by mass or more, and may be 14% by mass or less, 12% by mass or less, or 10% by mass or less.

[0022] The mold powder may further contain Al2O3. The content of Al2O3 in the mold powder may be 1% by mass or more, 1.5% by mass or more, or 2% by mass or more, and may be 10% by mass or less, 8% by mass or less, or 6% by mass or less.

[0023] The mold powder of the present embodiment has a solidification temperature of 1200°C or lower. From the viewpoint of further suppressing the bear, the solidification temperature of the mold powder is preferably 1190°C or lower, 1180°C or lower, or 1170°C or lower, and more preferably 1160°C or lower. The solidification temperature of the mold powder may be 1100°C or higher, 1110°C or higher, or 1120°C or higher.

[0024] The solidification temperature of the mold powder is measured by the following procedure. First, pour about 180 g of molten slag obtained by melting the mold powder into an iron container (inner diameter 40 mm Φ × height 80 mm), and while immersing the rotor (diameter 20 mm Φ × length 20 mm) of a B-type viscometer in the molten slag, heat it up to 1295 °C and hold it. Subsequently, after cooling by 10 °C, repeat the operation of rotating the rotor and measuring the torque due to the viscous resistance. Then, record the temperature T at which the torque overflows, and set the temperature T + 5 °C as the solidification temperature.

[0025] The mold powder of this embodiment has a crystallization temperature of 500 °C or higher and 660 °C or lower. The upper limit value of the crystallization temperature of the mold powder is preferably 650 °C or lower, 640 °C or lower, 630 °C or lower, 620 °C or lower, 610 °C or lower, or 600 °C or lower from the viewpoint of further suppressing cracks on the surface of the slab. The lower limit value of the crystallization temperature of the mold powder may be 505 °C or higher, 510 °C or higher, or 513 °C or higher.

[0026] The crystallization temperature of the mold powder is measured by the following procedure. First, melt about 20 g of the mold powder at 1400 °C for 3 minutes, then put it into a container containing about 700 mL of tap water and rapidly cool (water-cool) it to vitrify the mold powder. Subsequently, perform differential thermal analysis (DTA) on the vitrified mold powder starting from room temperature at a heating rate of 10 °C / min, and define the temperature at which crystals crystallize as the crystallization temperature. Note that the temperature at which crystals crystallize is defined as the temperature of the exothermic peak due to crystal crystallization.

[0027] As described above, the crystallization temperature of the mold powder of this embodiment means the temperature at which the mold powder vitrified by being rapidly cooled after melting crystallizes due to subsequent heating, and is distinguished from the temperature at which the molten mold powder (non-vitrified mold powder) is gradually cooled and crystallizes (such a temperature may also be called the crystallization temperature).

[0028] The difference between the solidification temperature and the crystallization temperature of the mold powder (solidification temperature - crystallization temperature) is preferably 480°C or higher, 490°C or higher, 500°C or higher, 510°C or higher, 520°C or higher, 530°C or higher, 540°C or higher, or 550°C or higher, more preferably 560°C or higher, from the viewpoint of further suppressing cracks on the surface of the slab. The difference between the solidification temperature and the crystallization temperature of the mold powder (solidification temperature - crystallization temperature) may be 720°C or lower, 700°C or lower, 680°C or lower, or 670°C or lower.

[0029] The solidification temperature and the crystallization temperature of the mold powder can be within the above ranges, for example, by setting the cuspidine ratio of the mold powder to 0.6 or higher and the total content of Na2O and Li2O to 6% by mass or higher, increasing the content of Li2O relatively, and making the composition such that the viscosity during melting (for example, the viscosity at 1300°C) is low.

[0030] The mold powder described above is used for continuous casting of steel. FIG. 1 is a schematic diagram showing the state of the mold powder in the mold. As shown in FIG. 1, the mold powder in the mold 1 exists as a molten layer 3 in a molten state on the molten steel 2 and as a powder layer 4 in a powder state on the molten layer 3. Further, on the surface (inner wall surface) of the mold 1, a bear 5 is formed by solidification of the once-molten mold powder or sintering and solidification of the mold powder.

[0031] By using the mold powder according to this embodiment, in the continuous casting of steel, it is possible to suppress the occurrence of cracks on the surface of the slab and the occurrence of bear in the mold 1. The reason will be explained with reference to Fig. 2 (a schematic diagram showing the main part P of Fig. 1). Near the mold 1, the molten layer 3 (once molten mold powder) solidifies to form a crystal layer 5a, and a glass layer 5b is formed in a region closer to the mold 1. When the mold powder according to this embodiment is used (Fig. 2 (b)), compared with the case where a conventional mold powder is used (Fig. 2 (a)), the crystal layer 5a of the mold powder in the mold 1 moves closer to the inner wall surface side of the mold 1. As a result, the molten layer 3 of the mold powder spreads, so that while maintaining slow cooling, the suppression of cracks on the surface of the slab due to improved lubricity by the mold powder and the suppression of the occurrence of bear are achieved simultaneously, the inventors speculate. That is, another embodiment of the present invention is a method of suppressing the occurrence of cracks on the surface of the slab and bear by using the above-described mold powder to move the crystal layer of the mold powder in the mold closer to the inner wall surface side of the mold and expand the molten layer of the mold powder.

[0032] The type of steel to be continuously cast is not particularly limited, but is preferably medium carbon steel. Medium carbon steel is steel having a carbon content of, for example, 0.08 to 0.20% by mass. Since medium carbon steel is a steel type in which cracks are likely to occur on the surface of the slab, by using the above-described mold powder in the continuous casting of medium carbon steel, the advantages of the mold powder (suppressing cracks on the surface of the slab and also suppressing the occurrence of bear) can be effectively exerted.

Examples

[0033] Hereinafter, the present invention will be described more specifically based on examples, but the present invention is not limited to the examples.

[0034] Mold powders of Examples and Comparative Examples having the slag components shown in Tables 1 to 3 were prepared. Note that only the mold powder of Comparative Example 6 contains 4% by mass of ZrO2 in addition to the components shown in Table 3. The mass ratio (CaO / SiO2) of the content of CaO to the content of SiO2, the cuspidine ratio, the solidification temperature, and the crystallization temperature of these mold powders are also shown in Tables 1 to 3.

[0035] (Effect of suppressing cracks on the slab surface) Molten steel (medium carbon steel) having a composition of C: 0.10% by mass, Si: 0.20% by mass, Mn: 0.50% by mass, P: 0.02% by mass, S: 0.01% by mass, and Al: 0.02% by mass was poured into a mold (thickness 250 mm, width 1800 mm), and continuous casting was carried out at a casting speed of 2.0 m / min while supplying the mold powders of Examples and Comparative Examples. If the number of slabs with cracks on the slab surface was 1 or less per 1 t (casting length of about 1000 m) of mold powder used, it was rated as "A", if it was 2 to 3, it was rated as "B", and if it was 4 or more, it was rated as "C", and the effect of suppressing cracks on the slab surface was evaluated. The results are shown in Tables 1 to 3.

[0036] (Effect of suppressing the occurrence of bear) After the above continuous casting was carried out, the thickness (maximum thickness) of the bear at the location where the thickness of the bear generated on the inner wall surface of the mold was the maximum was measured with a vernier caliper. If the measured maximum thickness of the bear was 10 mm or less, it was rated as "A", if it exceeded 10 mm and was 20 mm or less, it was rated as "B", and if it exceeded 20 mm, it was rated as "C", and the effect of suppressing the occurrence of bear was evaluated. The results are shown in Tables 1 to 3.

[0037]

Table 1

[0038]

Table 2

[0039]

Table 3

Explanation of Symbols

[0040] 1... Mold, 2... Molten steel, 3... Molten layer, 4... Powder layer, 5... Base, 5a... Crystal layer, 5b... Glass layer.

Claims

1. SiO ２ , CaO, F, Na ２ O, and Li ２ O-containing mold powder, wherein the minimum value among the content (mass%) of SiO ２ / 31.4, the content (mass%) of CaO / 58.6, and the content (mass%) of F / 10.0 is 0.6 or more, the total content of Na ２ O and Li ２ O is 6 mass% or more, the solidification temperature of the mold powder is 1200 °C or lower, and the crystallization temperature of the mold powder is 500 °C or higher and 660 °C or lower. Mold powder.

2. The mold powder according to claim 1, wherein the difference between the solidification temperature and the crystallization temperature is 490 °C or more.

3. The mold powder according to claim 1 or 2, which is used for continuous casting of medium carbon steel.

4. A method for suppressing the occurrence of cracks on the surface of a slab and bare in a mold in continuous casting of steel, comprising using, as the mold powder, SiO ２ , CaO, F, Na ２ O, and Li ２ O-containing mold powder, wherein the minimum value among the content (mass%) of SiO ２ / 31.4, the content (mass%) of CaO / 58.6, and the content (mass%) of F / 10.0 is 0.6 or more, the total content of Na ２ O and Li ２ O is 6 mass% or more, the solidification temperature of the mold powder is 1200 °C or lower, and the crystallization temperature of the mold powder is 500 °C or higher and 660 °C or lower. A method for suppressing cracks on the surface of the slab and generation of the bear by using mold powder.

5. The method according to claim 4, wherein the difference between the solidification temperature and the crystallization temperature is 490 °C or higher.

6. The method according to claim 4 or 5, wherein the steel is medium carbon steel.

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