JP2008043987A - Mold powder used for continuous casting of molten metal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance a heat-retention property on a molten metal surface inside a mold while reducing environmental contamination. <P>SOLUTION: The mold powder is a granulated mold powder in which an expandable graphite is blended. The bulk density thereof after granulated mold powder is thermally collapsed at the heating time and the carbon containing the expandable graphite is frazzled is smaller than the bulk density of the mixed material of raw material powder other than the carbon raw material, and is ≤0.65 g/cc. In this way, when the molten metal of steel or the like is continuously casting, it is compatibly possible to reduce environmental contamination and also to enhance the heat-retention property on the molten metal surface inside the mold. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a mold powder used for continuous casting of molten metal such as steel, and relates to a mold powder that achieves both low environmental contamination and high heat retaining properties of the mold surface.

  Mold powder for continuous casting of molten metal such as steel is in the form of powder or granules. When this raw powder is added to the mold, it melts on the molten metal surface and has a thickness of several to tens of millimeters. Form. The molten mold powder flows between the mold and the slab (solidified shell) to form a powder film. Among these, the powder film on the mold side is cooled and solidified, crystallizes or precipitates, moves to the lower part of the mold as the casting progresses, and is eventually discharged from the lower end.

In each of these processes, the mold powder plays various roles. Among them, the important role played by the raw powder layer is the insulation of the hot water surface.
Mold powder is roughly classified into powder and granule. In general, a powder product as disclosed in Patent Document 1 is advantageous in terms of heat retention because the bulk specific gravity can be reduced. On the other hand, the powder layer on the surface is likely to be scattered as dust and easily cause environmental pollution. On the other hand, the granule product is excellent in that it is difficult to reduce the bulk specific gravity and is inferior in heat retaining property, but on the other hand, dust scattering is small and environmental pollution is small.
JP 2004-98092 A

  That is, in the conventional mold powder, it has been difficult to achieve both the environmental pollution and the excellent heat retaining property of the mold surface. Therefore, in recent years, the importance of improving the working environment is increasing, and the cases of applying granule products are increasing. However, when a granule is applied, operation and quality problems due to the poor heat retention are often induced, and therefore, a mold powder with less dust and excellent heat retention has been demanded.

  The problem to be solved by the present invention is that when granulated mold powder is used so that dust scattering is small and environmental pollution is small, operation and quality problems due to poor heat retention are induced. There are many points.

  The present invention has been made as a result of investigation and research on a granular mold powder that solves the above-described problems, reduces environmental pollution, and is excellent in the heat retaining property of the mold surface.

That is, the mold powder used for continuous casting of the molten metal of the present invention is
A granular mold powder containing expansive graphite,
The granular mold powder collapses when heated,
Most importantly, the bulk specific gravity after the carbon containing expansive graphite is completely burned after the thermal collapse is smaller than the bulk specific gravity of the mixture of the raw material powders excluding the carbon raw material and not more than 0.65 g / cc. It is a feature.

  ADVANTAGE OF THE INVENTION According to this invention, in continuous casting of molten metals, such as steel, it can satisfy | fill both that environmental pollution is small and the heat retaining property of the hot_water | molten_metal surface in a casting_mold | template is high.

Hereinafter, the best mode as well as the mode for carrying out the present invention will be described in detail.
The inventor adds expansive graphite to a granular mold powder and thermally collapses it into a powder when heated in a mold. It was found that if the blending amount of expandable graphite is increased after suppressing pulverization, a very light foamed granular powder can be obtained after thermal collapse.

  The bulk specific gravity of the foamed granular powder was smaller than the bulk specific gravity of the mixture of the raw material powder excluding the carbon raw material even after burning the carbon containing the expandable graphite. This means that, when the expandable graphite is thermally expanded, a foaming action is generated, and a powdery sintered body having a high porosity is formed. The inventor confirmed by observation with a microscope or the like that the foamed granular powder generated after the thermal collapse was a powdery sintered body as described above.

That is, the invention according to claim 1 of the present application is
A granular mold powder containing expansive graphite,
The granular mold powder collapses when heated,
The bulk specific gravity after completely burning carbon containing expansive graphite after thermal collapse is smaller than the bulk specific gravity of the raw material powder mixture excluding the carbon raw material, and is 0.65 g / cc or less. Is.

  In the invention according to claim 1 of the present application, blending of expansive graphite, which is a basic matter of the present invention, and bulk specific gravity after completely burning carbon after thermal collapse are specified.

  Here, expansive graphite is graphite in which an acid such as sulfuric acid is impregnated between the laminated structures of graphite, and when the acid is volatilized by heating, the gap between the laminated structures widens and expands several tens to several hundreds times. It has characteristics.

  The bulk specific gravity after carbon containing expansive graphite is completely burned after thermal collapse means that the mold powder is heated for 45 minutes in an air atmosphere furnace at 800 ° C. The specific gravity obtained from the weight when the later expansive graphite and other carbon were completely burned, cooled to room temperature, and a 100 cc sample was filled in a cylindrical container having an outer diameter of 50 mm without vibration.

  Since the bulk specific gravity of general powdered mold powder is about 0.6 to 1.0 g / cc, the bulk specific gravity of 0.65 g / cc or less as defined in claim 1 is particularly lightly maintained among powder products. Corresponds to good mold powder. In the present invention, the bulk specific gravity after thermal collapse is more preferably 0.60 g / cc or less. The bulk specific gravity of 0.65 g / cc or less or 0.60 g / cc or less of the present invention is a large feature that it is a smaller (lighter) value than a mixture of raw material powders excluding carbon.

  The mold powder that satisfies the above-mentioned requirements defined in claim 1 of the present invention maintains the granular shape until it is conveyed into the mold, so that the generation of dust is reduced. In addition, since the powder is very light and has good heat retention in the mold, it has excellent heat retention. That is, in the present invention, the two requirements of small environmental pollution and excellent heat retention can be satisfied at a high level.

The invention according to claim 2 of the present application is
In the invention according to claim 1 of the present application,
The content of the binder for granulation is 1.5 to 5.0% by mass,
The content of the expandable graphite is 1.5 to 4.0% by mass.

  According to the inventor's research, it was found that when the binder for granulation is less than 1.5% by mass, more granules are broken and pulverized when fed into the mold. The inventor has found that when the granule is pulverized, it does not become a light powder after thermal collapse, and as a condition for further exerting the effect of the invention according to claim 1 of the present application, the binder content is 1. It was found that 5% by mass or more is necessary.

  On the other hand, if the binder content exceeds 5.0% by mass, the viscosity of the slurry before granulation becomes too high, and the moldability deteriorates, which is not preferable. According to the inventor's experiment, when the powdering rate in the mold is defined as the weight ratio of particles having a particle size of less than 0.21 mm, if the powdering rate can be suppressed to 25% or less, more preferably 20% or less, A small bulk specific gravity powder was obtained after disintegration.

  Further, in order to obtain a light powder by the thermal expansion action, it is necessary to contain 1.5% by mass or more of expandable graphite. On the other hand, if the content of expandable graphite exceeds 4.0% by mass, there is a problem that the mold powder is slowly hatched and melted, resulting in high costs.

The invention according to claim 3 of the present application is
In the invention according to claim 1 or 2 of the present application,
The total content of Na ₂ O, K ₂ O and Li ₂ O, which are alkali metal oxides, is 7% by mass or less,
The total content of carbonate is less than 10% by mass.

The inventor believes that the mold powder unmelted layer in the mold is effective in achieving both prevention of environmental pollution and ensuring heat retention in the form of a layered form in which the upper part is a granular layer and the lower part is a powder layer. I realized that it was to be kept stable. Then, the formation of alkali metal oxides in which Na ₂ O, K ₂ O and Li ₂ total content of O is more than 7% by mass, the powder layer powder layer sintering proceeds easily stable in is inhibited, When the amount of carbonate is 10% by mass or more, the lower powder layer is blown upward and a stable two-layer state cannot be maintained, and the invention according to claim 3 has been established.

The total content of Na ₂ O, K ₂ O and Li ₂ O is an alkali metal oxide, further preferably 3 wt% or less. The alkali metal oxide does not need to be contained, but even when it is not intended to be contained, usually about 0.5 to 2% by mass is contained as an impurity.

  Carbonate may not be blended, but it is preferable to contain 2% by mass or more because an anti-sintering effect due to a stirring action can be expected. A more preferred upper limit is 7% by mass. In addition, as carbonate, sodium carbonate, magnesium carbonate, calcium carbonate, strontium carbonate, etc. are used.

The invention according to claim 4 of the present application is
In the invention according to any one of claims 1 to 3 of the present application,
The carbon black content is 0.8% by mass or less, or is not contained.

  Since carbon black has a stable and strong action of suppressing hatching and melting, about 1 to 3% by mass is usually added to the mold powder. Since this carbon black is a fine powder, it has a strong environmental pollution effect.

  However, in the mold powder of the present invention, since the very light powder layer generated by thermal foaming has a high heat insulating action, hatching and melting are moderately suppressed even if the amount of carbon black is small. Furthermore, since the mold powder of the present invention has a small amount of alkali metal oxide, it is possible to control the hatching and melting with a small amount of carbon black by reducing the melting rate.

  Therefore, in the present invention, an environmentally friendly mold powder having a carbon black content of 0.8% by mass or less or containing no carbon black can be obtained. A more preferable value of the carbon black content upper limit is 0.5% by mass.

  Incidentally, there is a mold powder described in Japanese Patent No. 3128496 as a conventional invention similar to the present invention. Claim 6 of Japanese Patent No. 3128496 is similar to the present invention in that it is a granular mold powder containing expanded graphite.

  However, in Japanese Patent No. 3128496, there is no description about the bulk specific gravity important for heat retention and the binder content important for bulk specific gravity after thermal collapse of granules, and the improvement effect in terms of environmental pollution and heat retention is obvious. is not.

  Further, all examples of Japanese Patent No. 3128496 have a high content of alkali metal oxide, so that the powder layer below the granule layer in the mold is easy to sinter, inferior in heat retaining properties, and melt / reduced when carbon black is reduced. It is estimated that there is difficulty in hatch control.

Hereinafter, description will be given of the results of experiments performed to confirm the effects of the present invention.
Generally the chemical composition of the mold powder is composed mainly of CaO and SiO _2, together with CaO and SiO ₂ contained about 60 to 80 wt%, the ratio CaO / SiO ₂ (basicity) of CaO and SiO ₂ is It is in the range of about 0.6 to 1.8.

In addition, Al ₂ O ₃ , MgO, TiO ₂ , ZrO ₂ , Na ₂ O, Li ₂ O, B ₂ O ₃ , F, etc. are contained as regulators such as solidification temperature, viscosity or crystallization. Several mass% of aggregate such as carbon is added to the base material for the purpose of adjusting the hatching (melting) rate.

  In the case of a granule product, a binder (glue) is added to a slurry obtained by kneading these with water to form a granule. As the binder, CMC (carboxyl methyl cellulose) is widely used.

  Specific examples of the present invention and comparative examples in the mold powder having the general composition shown above are shown in Table 1 below.

A and B in Table 1 are examples that satisfy all claims 1 to 4 of the present application.
In these, the particle shape at room temperature is a granule (an extrusion-molded granule), and since it contains a sufficient amount of binder, the particle of the granule is strong and hardly broken. Therefore, almost no dust is produced until it is sprayed into the mold. Furthermore, since the alkali metal oxide content is low and the carbonate content is low, the upper granule layer and the lower powder layer are stably formed in the mold, and the generation of dust is also small in the mold. Moreover, since fine carbon black is not contained, the floating time of the generated dust is short. Due to these properties, Example A and Example B are mold powders with very little environmental contamination.

  At the same time, in Examples A and B, the bulk specific gravity (lab measurement value) after thermal collapse is 0.65 or less, and very light powder particles are formed near the molten steel surface in the mold. Is excellent. Such light powder particles are due to containing a sufficient amount of expandable graphite.

  In Examples A and B, since the binder is sufficiently contained, the ratio of the granular shape is maintained before the thermal collapse in the actual mold. Therefore, the bulk specific gravity after thermal collapse equivalent to the above laboratory measurement value can be obtained even in an actual mold.

Further, Example A and Example B are low in alkali metal oxide content and carbonate content, so that the upper granular layer and the lower powder layer are stably formed in the mold, and are excellent in heat retention. .
In Example B, the binder content was as high as 4% by mass close to the upper limit defined in claim 2, so the fluidity of the slurry before granulation was slightly poor and the moldability of the granules was somewhat difficult. .

C in Table 1 is an example satisfying claims 1 and 4 of the present application.
In Example C, since the particle shape at room temperature is a granule, the amount of dust generated at the stage until it is sprayed into the mold is small. However, since the content of the binder is 1% by mass and slightly less than 1.5% by mass, the proportion of the granule particles that are pulverized before being sprayed in the mold is slightly higher than in Examples A and B. There were many.

  In Example C, the carbonate content exceeded the upper limit of 10 mass% and was as large as 13 mass%, so that dust generation in the mold was slightly noticeable. Moreover, since Example C does not contain fine-grained carbon black, the floating time of the generated dust is short. From these characteristics, it can be seen that Example C is a mold powder with less environmental pollution, although it is inferior to Examples A and B.

  At the same time, in Example C, the bulk specific gravity (lab measurement value) after thermal collapse is 0.65 or less, and very light powder particles are formed in the vicinity of the molten steel surface in the mold. . Such light powder particles are due to containing a sufficient amount of expandable graphite.

  However, in Example C, the binder content is 1% by mass, which is slightly less than the lower limit of 1.5% by mass. Compared to Example B, the number was slightly higher. When granule particles are pulverized, the bulk specific gravity after thermal collapse is not as small as in laboratory measurements, so the heat retention when Example C is applied to an actual machine is compared to Example A or Example B. Somewhat inferior.

D in Table 1 is a comparative example that does not satisfy the claims of the present application.
Comparative Example D is a mold powder of a conventional granule that does not contain expansive graphite (an extrudate-shaped extruded granule). Although the environmental contamination is small, the bulk specific gravity is large, so that the heat retaining property is inferior.

E in Table 1 is also a comparative example that does not satisfy the claims of the present application.
Comparative Example E is a mold powder of a conventional type, and its bulk specific gravity is relatively small, so it is excellent in heat retention, but it is difficult to cause environmental pollution. Moreover, although the bulk specific gravity of the comparative example E belongs to a small class as a powder mold powder, it is not as small as the bulk specific gravity of Example A-Example C of this invention. This does not mean that the heat collapse referred to in the present invention is simply a phenomenon in which a granular product is collapsed, but rather than the original powder raw material mixture by collapsing while changing the foamed particle shape. This also confirms the phenomenon of collapse into a foamed granular powder having a small bulk specific gravity. This foam-like particle shape change is presumed to be caused by a gas released when the expandable graphite expands.

Next, Example A and Comparative Example E in Table 1 will be described with specific examples applied to continuous casting.
Example A and Comparative Example E were blended in the chemical composition (melted slag composition, not including carbon) shown in Table 2 below, and subjected to continuous casting. The casting conditions were as follows: mold size was 300 × 400 mm, casting speed was 0.5 m / min, and Al—Si killed plain steel with a carbon concentration of 0.10% by mass was cast.

The molten steel from the same ladle was branched into different strands and cast, and both mold powders were supplied to different strands to examine the influence on the slab quality.
The slab quality was evaluated by a method in which a slab sample having a length of 400 mm was taken in the longitudinal direction, the surface was pickled with hydrochloric acid, and the number of pinholes having a diameter of 1 mm or more observed on the surface was counted.

  As a result, the number of pinholes on the slab surface cast using Example A was reduced to 28 when the number of pinholes using Comparative Example E was set to 100. This is considered to be the manifestation of the heat retention improvement effect of the mold powder according to the present invention. Furthermore, it was confirmed that Example A had less dust generated during casting and less environmental pollution than Comparative Example E.

  The present invention is not limited to the above example, and it goes without saying that the embodiment may be appropriately changed within the scope of the technical idea described in each claim.

The present invention can be applied not only to continuous casting of steel as shown in the examples but also to continuous casting of pure Ni or the like.

Claims (4)

A granular mold powder containing expansive graphite,
The granular mold powder collapses when heated,
The bulk specific gravity after completely burning carbon containing expansive graphite after thermal collapse is smaller than the bulk specific gravity of the raw material powder mixture excluding the carbon raw material, and is 0.65 g / cc or less. Mold powder used for continuous casting of molten metal. The content of the binder for granulation is 1.5 to 5.0% by mass,
The mold powder used for continuous casting of molten metal according to claim 1, wherein the content of the expandable graphite is 1.5 to 4.0% by mass. The total content of Na ₂ O, K ₂ O and Li ₂ O, which are alkali metal oxides, is 7% by mass or less,
The mold powder used for continuous casting of molten metal according to claim 1 or 2, wherein the total content of carbonate is less than 10% by mass. The mold powder used for continuous casting of molten metal according to any one of claims 1 to 3, wherein the content of carbon black is 0.8 mass% or less or is not contained.