Mold powder

Abstract

To provide a mold powder that allows both improvement of melting rate of the mold powder and prevention of excessive increase in thickness of a molten mold powder layer to be sufficiently achieved.SOLUTION: The mold powder has a composition containing, in mass%, CaO and SiO2 in a total amount of 50.0% or more, Na2O in an amount of 10.0% or more, SrO in an amount of 7.0% or less, and T.C in an amount of 5.0% or less, the proportion existing as at least one carbonate selected from the group consisting of CaCO3, Na2CO3, and SrCO3 being 5.00% or less, the proportion existing as glass powder (soda-lime glass) being 3.0% or more, the basicity (CaO / SiO2) being 0.70 or more and 1.30 or less, and the viscosity at 1300°C being 0.15 Pa s or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a molding powder that achieves a stable melting rate. [Background technology]

[0002] In the continuous casting of molten metal, mold powder for continuous casting is supplied to coat the surface of the molten metal in the mold. Hereinafter, this mold powder for continuous casting will be simply referred to as "mold powder." The mold powder supplied into the mold forms a molten layer on the surface of the molten metal due to the heat from the molten metal, and the molten mold powder flows from the meniscus of the molten steel along the inner wall of the mold into the gap between the mold and the solidified shell, forming a film.

[0003] The mold powder is required to have the following properties for continuous casting of molten metal. First, the molten mold powder layer formed by melting the mold powder on the surface of the molten steel and the unmelted mold powder layer on top of that cover the surface of the molten steel, blocking contact with the air and providing the effect of preventing re-oxidation of the molten steel and maintaining its temperature. Second, since the molten mold powder needs to flow between the mold and the solidified shell and act as a lubricant, it is required that the mold powder is constantly supplied in an appropriate amount and that the melting rate is such that the molten mold powder pool has an appropriate thickness in accordance with the mold powder consumption rate. Third, the molten mold powder layer must absorb non-metallic inclusions that rise to the surface in the molten steel, and the absorption of the non-metallic inclusions must result in minimal changes in the physical properties of the molten mold powder (viscosity, melting temperature, solidification temperature, etc.). Fourth, the molten mold powder flows into the gap between the mold and the solidified shell, forming a uniform powder film that acts as a lubricant between the mold and the solidified shell. Depending on the properties of the steel being cast, slow cooling of the solidified shell may also be required. Fifth, the molten mold powder must have an appropriate viscosity and interfacial tension so that it is not entrained in the molten steel.

[0004] As described above, mold powders are required to possess many properties, and various technologies have been proposed to improve these properties. Patent Document 1 discloses mold powder containing up to 30 mass% CO2, and containing limestone (CaCO3) and sodium carbonate (Na2CO3) as raw materials containing CO2, to address issues such as lubrication between molten steel and the mold, heat transfer, and radiant heat loss (heat retention). Patent Document 2 also discloses mold powder in which the amount of Group 2 metal carbonate (magnesium carbonate, calcium carbonate, strontium carbonate, barium carbonate) and SiO2 derived from soda-lime glass are adjusted to prevent sintering of the mold powder and the occurrence of slag rim (slag bear). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 54-35129 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-135493 Summary of the Invention [Problem to be solved by the invention]

[0006] When the consumption rate of the mold powder decreases during continuous casting and there is a suspicion that lubrication will be impaired, it is common to increase the thickness of the molten mold powder layer to increase the melting rate of the mold powder. However, there are cases where the melting rate of the mold powder cannot be controlled, and in such cases the thickness of the molten mold powder layer increases, and even the unmelted mold powder layer melts, which can cause problems such as loss of heat retention due to an inability to block contact with air.

[0007] In the case of the molding powder described in Patent Document 1, the melting rate of the molding powder can be improved, but the thickness of the molten molding powder layer cannot be sufficiently prevented from increasing. Also, in the case of the molding powder described in Patent Document 2, either the thickness of the molten molding powder layer cannot be prevented from increasing, or the melting rate may decrease and insufficient flowability (lubricity) may occur, so it is not possible to sufficiently improve the melting rate of the molding powder and prevent the thickness of the molten molding powder layer from increasing at the same time.

[0008] In view of the above-mentioned problems, the present invention has an object to provide a molding powder that satisfies both an improvement in the melting rate of the molding powder and prevention of an increase in the thickness of the molten molding powder layer. [Means for solving the problem]

[0009] The inventors have investigated the causes of cases in which the thickness of the molten mold powder layer increases even when the same mold powder design and casting conditions are used. As a result, they discovered that some carbonates contained in the mold powder decompose at temperatures above 800°C, generating CO2, which acts to stir the inside of the mold. Therefore, depending on the stirring conditions, this can accelerate the melting of the mold powder and increase the thickness of the molten mold powder layer. Therefore, the inventors came up with the idea of ​​reducing the content of some carbonates in the mold powder. Meanwhile, to prevent the melting rate of the mold powder from decreasing too much by reducing the content of these carbonates, they ensured the melting rate by melting the Na2O in the mold powder at around 800°C.

[0010] The present invention is as follows. [1] In mass%, CaO and SiO2: 50.0% or more in total, Na2O: 10.0% or more, SrO: 7.0% or less, and TC: 5.0% or less, A composition comprising: The proportion of one or more carbonates selected from the group consisting of CaCO3, Na2CO3, and SrCO3 is 5.00% or less, The proportion of glass powder (soda-lime glass) is 3.0% or more, The basicity (CaO / SiO2) is 0.70 or more and 1.30 or less, A molding powder characterized by a viscosity of 0.15 Pa·s or less at 1300°C.

[0011] Hereinafter, the oxide of metal X (X) that constitutes the mold powder will be referred to as m O n Unless otherwise specified, the amount of metal X present in the mold powder (forms other than oxides include carbonates, fluorides, and simple metals) is expressed as an oxide-converted value. The composition (mass%) of the mold powder is the ratio of the metals X1 to X2 that make up the mold powder. n The total of the oxide equivalent values ​​of , the amount of TC (if present), and the amount of F (if present) is taken as 100, and the proportions of each are shown as composition (mass%). [Effects of the Invention]

[0012] According to the present invention, it is possible to sufficiently improve the melting rate of the molding powder while preventing the thickness of the molten molding powder layer from increasing. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail. In the following description, "%" refers to "% by mass" unless otherwise specified.

[0014] As mentioned above, the thickening of the molten mold powder layer is thought to be due to the decomposition of certain carbonates contained in the mold powder at temperatures above 800°C, generating CO2 and stirring the inside of the mold. Here, certain carbonates are those with melting points above 800°C, specifically CaCO3, Na2CO3, and SrCO3. Low-melting-point carbonates decompose immediately upon addition of the mold powder to generate CO2 gas, which has no stirring effect. However, high-melting-point carbonates gradually decompose upon addition of the mold powder to generate CO2 gas, which has a stirring effect within the mold. The stirring effect is not reproducible, resulting in inconsistencies. A strong stirring effect accelerates the melting of the mold powder, resulting in a thickening of the molten mold powder layer. For the above reasons, by setting an upper limit on the total content of CaCO3, Na2CO3 and SrCO3, the variation in the stirring effect due to the generation of CO2 gas is suppressed, and the variation in the melting amount of the molding powder is prevented.

[0015] On the other hand, by controlling the total amount of these carbonates, it is possible to suppress the variation in the melting amount of the mold powder, but the melting speed of the mold powder will relatively decrease. First, in order to improve the melting speed of the mold powder, it is considered to mix a large amount of Na2O. Furthermore, the inventors have focused on the fact that Na2O reacts with SiO2 at around 800°C and starts to melt the mold powder, and in this embodiment, in order to maximize the effect of improving the melting speed by Na2O, a glass powder (soda-lime glass) is used that mixes raw materials containing both Na2O and SiO2.

[0016] Here, in raw materials in which Na2O and SiO2 coexist, the melting point varies depending on the ratio of Na2O to the total of Na2O and SiO2. Glass powder is an industrial raw material containing SiO2, an oxide, as its main component, as well as about 10-18% Na2O, another oxide, and about 5-15% CaO, another oxide, and generates a liquid phase at about 800°C. In this way, glass powder has the effect of raising the melting start temperature of mold powder, and furthermore, unlike carbonates, it does not melt and generate CO2 gas, which stirs the molten steel.

[0017] Next, the conditions for the molding powder according to this embodiment and the reasons for them will be described.

[0018] [CaO and SiO2] CaO and SiO2 are components commonly used in mold powders and are also the main components of mold powders. Here, CaO refers to the Ca content (mass%) contained in the mold powder converted to CaO, and other components include CaCO3 and CaF2. In this embodiment, the total content of CaO and SiO2 is 50.0% or more. If the total content of these components is less than 50.0%, the proportions of other components will increase relatively, and interactions will occur between these components, which may result in variations in the melting rate of the mold powder. Preferably, the total content of CaO and SiO2 is 70.0% or more. Meanwhile, the upper limit of the total content of CaO and SiO2 is specifically determined by the content of the essential components described below.

[0019] The basicity (CaO / SiO2) is set to 0.70 or more and 1.30 or less. If the basicity is less than 0.70, the SiO2 concentration in the liquid phase increases in the initial stage of melting of the molding powder, increasing the viscosity of the entire molding powder, inhibiting mass transfer and slowing the melting rate of the molding powder. On the other hand, if the basicity exceeds 1.30, crystals tend to crystallize in the mold, reducing lubricity. Preferably, the basicity is 0.90 or more and 1.20 or less.

[0020] [Na2O] Na2O is a component that reduces viscosity and improves the melting rate of the molding powder. Therefore, the Na2O content is set to 10.0% or more. If the Na2O content is less than 10.0%, a liquid phase cannot be secured, resulting in a slower melting rate of the molding powder. Preferably, the Na2O content is 12.0% or more. There is no particular upper limit for the Na2O content, but if there is too much NaO, crystals containing Na2O may precipitate, and this crystallization or precipitation may result in a decrease in lubricity. Therefore, it is preferable that the Na2O content be 20.0% or less. The content of Na2O includes not only Na2O present alone, but also Na2CO3, glass powder, and the like.

[0021] [SrO] SrO has the effect of lowering the melting point of the molding powder and is added to adjust physical properties such as the melting point and viscosity of the molding powder. However, if the SrO content is too high, high-melting-point crystals containing Sr may crystallize, impairing lubricity. Therefore, the SrO content is set to 7.0% or less, and preferably 5.0% or less. Note that SrO is not an essential component for obtaining the effects of the present invention, so it may be 0%.

[0022] [TC] TC refers to the total content (mass%) of C contained in the molding powder. Examples of C contained in the molding powder include inorganic substances such as carbonates and carbon black, and organic substances such as wood chips and stearic acid. C is contained for purposes such as melting rate control and heat source, and can be blended according to each purpose. If the TC is 5.0% or less, these purposes can be achieved without impairing the effects of the present invention. Note that, since the above-mentioned carbonates hinder the control of the melting rate, the TC including the carbonates may be 0%.

[0023] [CaCO3, Na2CO3 and SrCO3] At least a portion of the CaO, Na2O, and SrO may exist as CaCO3, Na2CO3, and SrCO3, respectively. These carbonates have melting points of 800°C or higher. As mentioned above, when the mold powder is poured into the mold, these carbonates decompose at high temperatures of 800°C or higher to generate CO2, which then stirs the molten steel in the mold. If this stirring effect is significant, it further accelerates the melting of the mold powder, causing the molten mold powder layer to thicken. Therefore, the proportion of one or more carbonates selected from the group consisting of CaCO3, Na2CO3, and SrCO3 present in the form of a molten mold powder is set to 5.00% or less, preferably 3.00% or less.

[0024] [Glass powder (soda-lime glass)] As mentioned above, NaO, when coexisting with SiO, begins to melt at around 800°C. Therefore, if at least a portion of the NaO is contained as glass powder (soda-lime glass), melting begins at around 800°C, ensuring a liquid phase, and improving the melting rate of the mold powder from a kinetic standpoint. To fully achieve this effect, some or all of the NaO and SiO contained in the mold powder must be present as glass powder, and the proportion of glass powder relative to the total amount of mold powder must be 3.0% or more. If the proportion of glass powder is less than 3%, a sufficient liquid phase cannot be ensured, and the melting rate of the mold powder cannot be sufficiently improved. Preferably, the proportion of glass powder is 4.0% or more. On the other hand, there is no particular upper limit for the proportion of glass powder, but if the proportion of glass powder is too high, the mold powder may sinter in conjunction with other components, causing slag bears to grow and hindering the flow of the mold powder. Therefore, the proportion of glass powder is preferably 20.0% or less.

[0025] The following components are also contained in conventional molding powders, and may be contained as necessary, provided that the effects of sufficiently improving the melting rate of the molding powder and preventing the thickness of the molten molding powder layer from increasing at the same time are not impaired.

[0026] [MgO] MgO has the effect of lowering the melting point of the molding powder and is added to adjust the physical properties of the molding powder, such as the melting point and viscosity. If the MgO content is 5.0% or less, these purposes can be achieved within a range that does not impair the effects of the present invention.

[0027] [Al2O3] Al2O3 is a component that increases viscosity and has the effect of improving the uniformity of the flow of mold powder, so Al2O3 may be contained in an amount of 10.0% or less.

[0028] [LiO] Li2O has the effect of lowering the melting point of the molding powder and is added to adjust the physical properties of the molding powder, such as the melting point and viscosity. If the Li2O content is 1.0% or less, these purposes can be achieved within a range that does not impair the effects of the present invention.

[0029] [F] F has the effect of lowering the melting point of the molding powder and is added to adjust physical properties such as the melting point and viscosity of the molding powder. If the F content is 10.0% or less, these purposes can be achieved within a range that does not impair the effects of the present invention. Note that F is added to the molding powder in the form of CaF2, NaF, Na3AlF6, etc., but in this embodiment, the content (mass%) of element F is used in all cases.

[0030] As other components, B2O3 may be contained in part depending on the steel type and purpose. Furthermore, FeO may be contained as an unavoidable impurity.

[0031] [Viscosity at 1300°C (Pa s)] Next, we will explain the physical properties of the mold powder. The higher the viscosity at 1300°C, the slower the mass transfer in the semi-molten state, and the longer the melting time, making it difficult to guarantee flowability. For these reasons, the viscosity at 1300°C should be 0.15 Pa·s or less. Preferably, the viscosity at 1300°C should be 0.13 Pa·s or less. On the other hand, there is no particular lower limit for the viscosity at 1300°C, but considering the uniformity of flow of the mold powder, it is preferable to set it to 0.07 Pa·s or more.

[0032] Viscosity can be measured using the rotating cylinder method. Specifically, the mold powder to be measured is placed in a crucible and pre-melted at 1400°C for 10-15 minutes. Then, the crucible is placed in a vertical tubular furnace (with a SiC heating element). The rotor of a cone-and-plate rotational viscometer is immersed in the molten powder and allowed to stabilize at 1300°C for 30 minutes. The rotor is then rotated, and the torque due to viscous resistance is measured to determine the viscosity. It is important to calibrate the viscometer with a standard viscosity liquid beforehand.

[0033] [Applicable steel type] While the mold powder of this embodiment can be used with any steel type, it is particularly effective with steels in which the theoretical proportion of γ phase does not reach 90% or more at any temperature below the liquidus temperature and above 1000°C. These steels have a low molten powder attraction effect between the solidified shell and the mold wall due to volumetric shrinkage during the transformation from δ phase to γ ​​phase, resulting in significantly low mold powder inflow. Therefore, the present invention is particularly effective. Specific examples of the composition include carbon steels with 0.07% or less C and 2.5% or more Si, carbon steels with 0.01% or less C, 0.3% or more Al, and 1.0% or more Si, and stainless steels with 10% or more Cr and a Cr / Ni ratio of 1.6 or more. [Example]

[0034] An example of the present invention will be described. The conditions in the example are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0035] First, to evaluate the variation in the melting rate of the mold powder, a φ50 mm graphite ring was floated on molten pig iron at 1530°C, and 30 g of the mold powder shown in Table 1 was added inside the graphite ring. The molten iron surface was then observed, and the time until the entire surface became molten was measured. This melting test was repeated five times, and the average value and variation (maximum value - minimum value) of the five melting tests were evaluated. The melting behavior was evaluated as × if the average melting time exceeded 45 seconds or the variation exceeded 5 seconds, and as ⊚ otherwise. In other words, if the average melting time exceeded 45 seconds, the mold powder melting rate was evaluated as low, and if the variation in the melting time exceeded 5 seconds, it was evaluated as possibly being unable to control the mold powder melting rate, resulting in the molten mold powder layer becoming thicker.

[0036] Next, a casting test was conducted using a continuous casting machine. Specifically, 150 tons or more of molten steel with a C concentration of 0.05% and a Si concentration of 2.5% by mass was prepared and poured into a mold measuring 900 to 1300 mm x 250 mm, and the mold powder shown in Table 1 was added. After secondary cooling, a cast slab was produced. The casting speed was 1.0 to 1.2 m / min.

[0037] The amount of mold powder used was then measured, and the mold powder consumption per ton of molten steel was calculated. The thickness of the molten mold powder layer in the mold was measured approximately six times every 10 minutes while casting at least 150 tons of molten steel. If a thickness of 20 mm or more was measured at any one time, it was evaluated as having enlarged. A mold powder consumption of less than 0.30 kg / ton or an enlarged molten mold powder layer was evaluated as "x." A mold powder consumption of less than 0.40 kg / ton without enlargement of the molten mold powder layer was evaluated as "good," and a mold powder consumption of 0.40 kg / ton or more was evaluated as "excellent." The test results are shown in Table 1. In other words, a low mold powder consumption was evaluated as a slow mold powder melting rate and poor lubricity.

[0038] [Table 1]

[0039] The underlined parts in the table indicate conditions that deviate from the conditions of the present invention. The "carbonate" in the table represents the total content of CaCO3, Na2CO3, and SrCO3. The "viscosity at 1300°C" in the table is the viscosity measured by the rotating cylinder method described above before conducting these tests. As shown in Table 1, Examples 1 to 15 all achieved good results in both the melting test and the casting test.

[0040] Comparative Examples 1 to 3 are examples in which a large amount of carbonates (CaCO3, Na2CO3, SrCO3) was contained. In the melting test, the molten iron was stirred by CO2 gas generated from the carbonates, so the melting rate of the mold powder could not be controlled, and the melting time varied. In the casting test, the melting rate became excessively high due to the stirring of the molten steel, so the molten mold powder layer became thicker.

[0041] Comparative Example 4 is an example in which the proportion of glass powder (soda-lime glass) present in the mold powder was too low. In the melting test, the proportion that became liquid at about 800°C was too low, so the melting start temperature could not be sufficiently increased, and the melting time was too long. In addition, in the casting test, the melting rate was low, so the amount of mold powder consumed was also low.

[0042] Comparative Example 5 is an example in which the amount of Na2O in the molding powder was insufficient. In the melting test, the liquid phase could not be sufficiently secured, so the melting initiation temperature could not be sufficiently increased, and the melting time was too long. In addition, in the casting test, the melting rate was low, so the amount of mold powder consumed was also small.

[0043] Comparative Example 6 is an example in which the viscosity of the molding powder was too high. In the melting test, the mass transfer in the semi-molten state was slow, and the melting time was too long. In the casting test, the melting rate was low, so the amount of molding powder consumed was also small.

Claims

[Claim 1] In mass%, CaO and SiO 2 : 50.0% or more in total, Na 2 O: 10.0% or more, SrO: 7.0% or less, and T. C: 5.0% or less, A composition comprising: CaCO 3 , Na 2 CO 3 and SrCO 3 The proportion of one or more carbonates selected from the group consisting of: is 5.00% or less, The proportion of glass powder (soda-lime glass) is 3.0% or more, Basicity (CaO / SiO 2 ) is 0.70 or more and 1.30 or less, A molding powder characterized by having a viscosity at 1300°C of 0.15 Pa·s or less.

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