Exothermic mold powder

Abstract

To provide a heat-generating mold powder that is excellent in suppressing slag bearing and is easy to crystallize even under a rapid cooling condition.SOLUTION: A heat-generating mold powder comprises, as a raw material blend: 1.0 mass% or more but less than 5.0 mass% of metal; and 0.1 mass% or more but less than 3.0 mass% of calcium peroxide, wherein 15.0 mass% or less (including zero) of Na2O is included as a chemical composition of the raw material.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to an exothermic mold powder suitable for continuous casting of steel.

Background Art

[0002] In the continuous casting of steel, molten steel stored in a tundish is poured into a mold through a submerged nozzle and cooled and solidified, and the solidified shell (solidification shell) is continuously drawn downward from the mold using rolls to continuously produce slabs, blooms, billets, and other various-shaped cast slabs. A powdery or granular mold powder is introduced onto the surface of the molten steel in the mold. The mold powder is melted by the heat of the molten steel (hereinafter, the molten mold powder in the molten state is referred to as "molten slag"), forms a molten slag layer to cover the surface of the molten steel, and the molten slag flows into the space between the solidification shell and the mold and is discharged and consumed in parallel with the solidification shell. The main roles of the mold powder from introduction to consumption are as follows. (1) Ensuring lubrication between the solidification shell and the mold (2) Controlling the heat flux from the solidification shell to the mold (controlling the cooling rate of the solidification shell) (3) Absorbing non-metallic inclusions floating from the molten steel and purifying the molten steel (4) Keeping the temperature of the molten steel surface (5) Preventing oxidation of the molten steel surface

[0003] Among these roles, for the purpose of improving "(4) keeping the temperature of the molten steel surface", there are those that utilize the oxidation and exothermic reaction of a metal or the like as an exothermic material and an oxidizing agent. This is called an exothermic mold powder.

[0004] Patent Document 1 discloses an exothermic front powder for continuous casting (a type of exothermic mold powder) containing 3 to 20% by mass of a metal or alloy and 3 to 15% by mass of an alkali metal nitrate as an oxidizing agent.

Prior Art Documents

Patent Documents

[0005] Patent Document 1 Japanese Patent Application Laid-Open No. 2019-136766 Patent Document 2 Japanese Patent Application Laid-Open No. 2004-001017 Summary of the Invention Problems to be Solved by the Invention

[0006] The nitrate disclosed in Patent Document 1 is useful as an oxidizing agent. However, nitrates are hygroscopic. When the exothermic mold powder absorbs moisture during storage or the like, the nitrate solidifies. When the nitrate solidifies, the uniform dispersibility decreases, so that the function of the nitrate as an oxidizing agent decreases, the calorific value becomes insufficient, and melting failure occurs, and slag bear (also called slag rim) may adhere to the inner circumference of the mold at the upper part of the meniscus. When slag bear occurs and grows, it may cause non-uniform inflow of slag between the mold and the solidified shell, and slab cracking may occur due to non-uniform heat extraction. Therefore, for the exothermic mold powder to cool the uniform solidified shell, suppression of slag bear is required.

[0007] In addition, medium carbon steel has particularly high crack sensitivity. Therefore, the exothermic mold powder for medium carbon steel needs to quickly prevent excessive heat extraction from the slab at the upper part inside the mold, and to quickly generate crystals between the mold and the solidified shell, that is, slow cooling characteristics due to a high crystallization temperature and a high crystallization rate are required. In order to realize such characteristics, a composition with low Na2O may be targeted (Patent Document 2, etc.). However, in such a low Na2O composition, it is difficult to add nitrates containing Na such as sodium nitrate.

[0008] The present disclosure has been made in view of the above actual situation, and an object thereof is to provide an exothermic mold powder that is excellent in suppressing slag bear and is likely to crystallize even under rapid cooling conditions. Means for Solving the Problems

[0009] One aspect of the present disclosure is As a raw material composition, it contains 1.0% by mass or more and less than 5.0% by mass of a metal and 0.1% by mass or more and less than 3.0% by mass of calcium peroxide. The exothermic mold powder is characterized in that its chemical composition of the raw materials contains 15.0% by mass or less (including zero) of Na2O.

[0010] The exothermic mold powder according to one aspect of the present disclosure is excellent in suppressing slag bearing and crystallizes even under rapid cooling conditions. Therefore, it is excellent in the uniformity of cooling of the solidified shell and can quickly prevent excessive heat extraction from the slab at the upper part inside the mold, that is, it has excellent slow cooling characteristics and can improve the slab quality. Therefore, the exothermic mold powder according to one aspect of the present disclosure is particularly suitable for the continuous casting of medium carbon steel.

[0011] In one aspect of the present disclosure, As the chemical composition, it is preferable to contain 4.0% by mass or less (including zero) of the Na2O. The crystallization rate becomes faster, it crystallizes even under rapid cooling conditions, and it has excellent slow cooling characteristics.

Mode for Carrying Out the Invention

[0012] Hereinafter, preferred embodiments of the present disclosure will be described in detail. It should be noted that the embodiments described below do not unduly limit the content of the present disclosure described in the claims, and not all of the configurations described in the embodiments are essential as the solution means of the present disclosure.

[0013] The exothermic mold powder of this embodiment contains, as a raw material blend, 1.0% by mass or more and less than 5.0% by mass of a metal and 0.1% by mass or more and less than 3.0% by mass of calcium peroxide, and contains 15.0% by mass or less (including zero) of Na2O as the chemical composition of the raw material. The exothermic mold powder of this embodiment is excellent in suppressing slag bearing and crystallizes even under rapid cooling conditions. Therefore, it is excellent in the uniformity of cooling of the solidified shell and can quickly prevent excessive heat extraction from the slab at the upper part inside the mold, that is, it has excellent slow cooling characteristics and can improve the slab quality. Therefore, the exothermic mold powder of this embodiment is particularly suitable for the continuous casting of medium carbon steel.

[0014] <Metal> The exothermic mold powder of this embodiment contains, as a raw material blend, 1.0% by mass or more and less than 5.0% by mass of a metal. The metal functions as a heat generating material and includes alloys, and examples thereof include one or more selected from Si, Al, Ca-Si, Al-Mg, Al-Ca-Mg, Fe-Si, etc. The content of the metal is preferably 1.0% by mass or more and 4.0% by mass or less, and more preferably 1.0% by mass or more and 2.0% by mass or less. When the content of the metal is within a suitable range, the calorific value can be maximized. The form of the metal is not particularly limited, and examples thereof include powders and granules.

[0015] <Calcium peroxide (CaO2)> The exothermic mold powder of this embodiment contains calcium peroxide in an amount of 0.1% by mass or more and less than 3.0% by mass as a raw material blend. Calcium peroxide functions as an oxidizing agent for metals. Since it has low hygroscopicity, fusion between raw materials hardly occurs, and it is uniformly dispersed by kneading. Also, because its decomposition temperature is lower than that of sodium nitrate, it oxidizes metals rapidly from a low temperature. Therefore, the oxidation of metals and the exothermic reaction proceed uniformly from a low temperature, suppressing the formation of slag bear. The form of calcium peroxide may be a pure substance or a preparation with auxiliary raw materials added to make handling safe and easy. Various commercially available products can be used as the preparation. In the case of a preparation, the content of calcium peroxide is converted to a pure substance. If the content of calcium peroxide is less than 0.1% by mass, oxidation and heat generation are insufficient. If it is 3.0% by mass or more, oxidation and heat generation become excessive, and the molten slag layer becomes undesirably thick. The content of calcium peroxide is preferably 0.5 to 1.0% by mass, and more preferably 0.6 to 0.9% by mass.

[0016] <na2o> The exothermic mold powder of the present embodiment contains Na2O of 15.0 mass% or less (including zero) as the chemical composition of the raw material. As a result, the exothermic mold powder has a faster crystallization rate, can crystallize even under rapid cooling conditions, and can quickly prevent excessive heat extraction from the slab at the upper part inside the mold. That is, it has excellent slow cooling characteristics and can improve the slab quality. This is particularly suitable for the continuous casting of medium carbon steel. The content of Na2O is preferably 4.0 mass% or less (including zero), and more preferably 3.5 mass% or less (including zero). In addition, calcium peroxide is preferable because neither the pure substance nor the preparation basically contains Na2O.

[0017] <Main raw material> The main raw materials constituting the exothermic mold powder of the present embodiment are not particularly limited as long as they are generally used for mold powder. For example, Portland cement of CaO·SiO2-based raw materials, limestone, quicklime, calcium silicate, synthetic calcium silicate, wollastonite, phosphorus slag, blast furnace slag, dicalcium silicate, sodium carbonate, calcium carbonate, perlite, fly ash, glass powder, silica fume, silica flower, silica sand, silica stone powder, diatomaceous earth, feldspar, etc. can be mentioned. The mass ratio (CaO / SiO2) of the CaO·SiO2-based raw materials is not particularly limited as long as it is generally used for mold powder.

[0018] <Auxiliary raw material> The exothermic mold powder of this embodiment may contain, as auxiliary raw materials, flux raw materials, carbon raw materials, and / or other raw materials generally used in mold powders. The flux raw materials have the role of adjusting the softening point, viscosity, and / or crystallization rate. For example, fluoride raw materials such as sodium fluoride, lithium fluoride, cryolite, fluorite (calcium fluoride), magnesium fluoride, carbonate raw materials such as sodium carbonate, lithium carbonate, magnesium carbonate, manganese carbonate, aluminum carbonate, magnesium carbonate, boric acid, borax, colemanite, etc. can be used. The carbon raw materials have the role of adjusting the slagging rate of the exothermic mold powder. For example, coke, graphite, carbon black, etc. can be used. As other raw materials, magnesia, alumina, etc. can be used. Also, trace amounts of Fe2O3, P2O5, S, etc. are allowed as inevitable components.

[0019] <Form of the exothermic mold powder> The form of the exothermic mold powder of this embodiment is not particularly limited as long as it is generally used in mold powders. For example, powders, extruded granules, hollow spray granules, agglomeration by stirring, etc. can be used.

Examples

[0020] Hereinafter, the examples of the present disclosure will be described in detail.

[0021] 1. Preparation of samples Calcium silicate of SiO2·CaO-based raw material as the main raw material, fluoride raw material, carbonate raw material, carbon raw material, alumina raw material, and / or magnesia as other raw materials as auxiliary raw materials, calcium peroxide or nitrate raw material as the oxidant, and metallic silicon as the metal were kneaded to prepare samples of the exothermic mold powder. Powders were used for all raw materials. The formulations of the raw materials of the samples and the content of Na2O in the chemical composition of the raw materials are shown in Tables 1 to 6.

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

[0022] In Examples 1 to 5 and Comparative Examples 1 and 2 of Table 1, the content of metallic silicon was fixed at 1.0% by mass, and the content of calcium peroxide was varied. In Examples 6 to 10 and Comparative Examples 3 and 4 of Table 2, the content of metallic silicon was fixed at 2.0% by mass, and the content of calcium peroxide was varied. In Examples 11 to 15 and Comparative Examples 5 and 6 of Table 3, the content of metallic silicon was fixed at 3.0% by mass, and the content of calcium peroxide was varied. In Examples 16 to 20 and Comparative Examples 7 and 8 of Table 4, the content of metallic silicon was fixed at 4.0% by mass, and the content of calcium peroxide was varied. In Examples 21 to 28 and Comparative Examples 9 and 10 of Table 5, the content of Na2O in the chemical composition of the raw materials was reduced. In Comparative Examples 9 and 10, sodium nitrate was used as an oxidizing agent. Table 6 shows Comparative Examples 11 to 16. In Comparative Examples 11 and 12, conventional sodium nitrate was used. Note that Comparative Example 11 was used as a criterion for evaluating the heat generation start time, as described later. In Comparative Examples 13 and 14, the content of Na2O in the chemical composition of the raw materials was increased. In Comparative Examples 15 and 16, the content of metallic silicon was increased to 5.0% by mass.

[0023] 2. Measurement and Evaluation Methods For the above samples, the following measurements and evaluations were performed.

[0024] <Heat Generation Start Time> The heat generation start time from the start of heating of the sample to the start of heat generation was measured by differential thermal analysis. That is, a crucible filled with 16 g of the sample and a crucible filled with 16 g of alumina fine powder as a reference sample were simultaneously inserted into an electric furnace at 900 °C, the differential thermal change was recorded, and the heat generation start time was determined.

[0025] A slow heat generation start time indicates that the oxidation and heat generation reaction of the metal are delayed, and slag bear is likely to be generated. A fast heat generation start time indicates that the melting of the powder is fast. If there is a difference of ±60 seconds or more compared to the reference Comparative Example 11, it is unacceptable (×) because it is too slow or too fast. If there is a difference of more than ±15 seconds and less than 60 seconds, it is acceptable (△) because it is slightly slow or slightly fast but there is no practical problem. If the difference is less than ±15 seconds, it is evaluated as good (〇) because it is equivalent to the reference Comparative Example 11.

[0026] <Melting property> Pig iron was melted in a high-frequency induction furnace, 400 g of the sample was sprayed onto the molten iron at 1500 °C, and the melting property of the sample was evaluated by visually observing the occurrence status of sintered lumps during the melting process.

[0027] It shows that the smaller the ratio of the sintered lumps in the decarburized state in the unmelted powder, the better the melting property. Observing from directly above in the furnace, when the area ratio of the unmelted powder to the molten slag is 1:1, if the ratio of the sintered lumps in the decarburized state in the unmelted powder is less than 25%, it is evaluated as good (〇). If it is 25% or more and less than 50%, it is acceptable (△) because there is no practical problem. If it is 50% or more, it is evaluated as unacceptable (×).

[0028] <Crystallization rate> The crystallization rate of the sample was evaluated from the crystal formation status in the slag film under rapid cooling conditions. That is, the sample was melted in an electric furnace at 1300 °C to form molten slag, a water-cooled SUS pipe was immersed in the molten slag, and a slag film was formed on the outer periphery of the SUS pipe. The immersion times were 10 seconds and 30 seconds.

[0029] The larger the proportion (crystalline area ratio) of the crystal structure formed on the surface of the slag film occupies the entire slag film, the faster the crystallization rate, that is, the easier it is to crystallize, indicating a preferable situation. When the crystalline area ratio is 50% or more after 10 seconds of immersion, the crystallization rate is fast (〇); when it is 50% or more after 30 seconds of immersion, it is acceptable (△) because there are no practical problems; when it is less than 50% after 30 seconds of immersion, the crystallization rate is slow (×) as evaluated.

[0030] <Overall Evaluation> For the overall evaluation, when all of the above three evaluations are 〇, it is excellent (◎); when there are two 〇 and no ×, it is good (〇); when there is one or less 〇 and no ×, it is acceptable (△); when there is one or more ×, it is unacceptable (×).

[0031] 3. Measurement and Evaluation Results The measurement and evaluation results are shown in Tables 1 to 6.

[0032] From Tables 1 to 5, in all examples, the heat generation start time was better compared to Comparative Examples 1 to 10 that did not contain calcium peroxide or contained 3.0 mass% or more of calcium peroxide in the raw material formulation. Therefore, in the raw material formulation, the calcium peroxide content is 0.1 mass% or more and less than 3.0 mass%, preferably 0.5 to 1.0 mass%, and more preferably 0.6 to 0.9 mass%. On the other hand, in Comparative Examples 1, 3, 5, and 7 that did not contain calcium peroxide, the heat generation start time was too slow, and in Comparative Examples 2, 4, 6, and 7 that contained 3.0 mass% or more of calcium peroxide, the heat generation start time was too fast.

[0033] From Table 5, in Examples 21 to 28 where the content of Na2O in the chemical composition of the raw material was reduced, the crystallization rate was good in all cases. On the other hand, in Comparative Examples 13 and 14 (Table 6) where the content of Na2O was increased, the crystallization rate was inferior. Therefore, in the chemical composition of the raw material, the content of Na2O is 15.0 mass% or less (including zero), preferably 4.0 mass% or less (including zero), and more preferably 3.5 mass% or less (including zero).

[0034] From Tables 1 to 5, the heat generation start time was good in all examples. On the other hand, Comparative Examples 15 and 16 (Table 6) in which the content of metallic silicon was increased resulted in poor meltability. Therefore, as the raw material formulation, the content of metallic silicon is 1.0 mass% or more and less than 5.0 mass%, preferably 1.0 mass% or more and 4.0 mass% or less, and more preferably 1.0 mass% or more and 2.0 mass% or less.

[0035] Although the present embodiment has been described in detail as above, those skilled in the art will easily understand that many modifications can be made without substantially departing from the novel matters and effects of the present disclosure. Therefore, all such modified examples are included in the scope of the present disclosure. For example, in the specification, a term described at least once together with a different term that is broader or synonymous can be replaced with that different term anywhere in the specification. Also, the configuration and operation of the manufacturing apparatus and the like of the present embodiment are not limited to those described in the present embodiment, and various modifications are possible.

Claims

1. As a raw material formulation, it contains 1.0% by mass or more and less than 5.0% by mass of a metal and 0.1% by mass or more and less than 3.0% by mass of calcium peroxide, As the chemical composition of the raw material, it contains Na of 9.2 mass% or less (including zero). ２ including The metal is one or more selected from Si, Al, Ca - Si, Al - Mg, Al - Ca - Mg, and Fe - Si, and is a heat - generating mold powder.

2. In the heat - generating mold powder according to Claim 1, As the chemical composition, the Na is 4.0 mass% or less (including zero). ２ An exothermic mold powder characterized by containing O.

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