JPS6356019B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a powder for coating the molten metal surface in a tundish or mold in continuous steel casting. As is well known, in continuous steel casting, when pouring molten steel from a ladle into a mold, it is usual to first pour the molten steel from the ladle into an intermediate container (tundish), and then pour it from the tundish into the mold. be.
In a tandate, the temperature of the molten steel surface is prevented from decreasing, and the deoxidation products floating on the surface of the molten steel, air oxides called scum, and suspended matter mixed with converter slag, etc., are absorbed or The surface of molten steel is coated with powder mainly composed of oxide powder for the purpose of diluting it to make it relatively harmless and also adsorbing non-metallic inclusions suspended in molten steel. is being carried out. Also, in molds, the surface of the molten metal is coated with powder mainly composed of oxide powder, in the same manner as described above, for the same purpose as mentioned above and for the purpose of lubricating the space between the slab and the inner wall surface of the mold. In order to efficiently achieve the above-mentioned purpose, it is necessary to appropriately adjust the physical properties of such powder for coating the hot water surface, such as viscosity, softening temperature, slag temperature, fluidity, and basicity. The composition of the powder must be adjusted so that it is unlikely to cause melting damage to firebricks, tundish nozzles, etc. From these points of view, the powder used in conventional continuous casting has a base material of CaO-SiO ₂ -Al ₂ O ₃ and flux components such as CaF ₂ and NaF.
Basicity is 0.8~ by adding appropriate amount of carbon as aggregate.
1.3. Generally, the softening temperature is adjusted to 1050 to 1250°C and the viscosity is adjusted to 2.5 to 100 poise (1300°C) depending on the purpose of use. By the way, in general, in molten steel with a high Al content, the Al in the molten steel and the above-mentioned floating substances such as MnO, FeO, Fe ₂ O ₃ , SiO ₂ , P ₂ O ₅ , etc. floating on the molten steel surface are easily reduced. Due to the reduction reaction between oxides
It is known that Al ₂ O ₃ is generated and this Al ₂ O ₃ is dispersed and suspended in the molten steel, causing contamination of the molten steel. Therefore, if a powder like the one mentioned above is used in a tundish with a particularly large amount of suspended matter, these suspended matter will be diluted and the aforementioned reduction reaction will occur.
The amount of Al ₂ O ₃ produced is reduced and the cleanliness is improved. However, for products such as Al-killed steel sheets for deep drawing and low-carbon Al-killed steel for bending, which require higher cleanliness, even if the powder described above is used, the cleanliness is sufficiently high. The reality is that we are not getting it. Furthermore, in continuous casting of these Al-killed steels, non-metallic inclusions such as Al ₂ O ₃ adhere and solidify inside the immersion nozzle (tundish nozzle) for injecting molten steel from the tundish into the mold. The nozzle often becomes clogged, but the reality is that this problem has not been fundamentally solved even when the powder described above is used. On the other hand, in the case of killed steel (Al≦0.01%), which is mainly composed of Si-Mn and contains almost no Al, the Al content is extremely small, so the reduction reaction between Al and the floating matter on the hot water surface causes Al Although it is thought that there is little risk of ₂ O ₃ being generated, the resulting steel still contains MnO―SiO ₂ , 3MnO―Al ₂ O ₃ ―
Trace amounts of non-metallic inclusions such as 3SiO ₂ and CaO-Al ₂ O ₃ remain, which may make it difficult to keep the oxygen value within a certain limit.
Even in killed steel with a low Al content, clogging of the tundish nozzle due to adhesion of nonmetallic inclusions occurred in the same manner as described above. The present invention has been made in view of the above circumstances, and aims to provide powder for continuously casting molten metal surface coating, which can effectively prevent nozzle clogging and produce slabs with less non-metallic inclusions. It is something. The inventor of the present invention has conducted various studies on the above-mentioned problems, and has found that the above-mentioned problems occur because there is a fundamental problem with the components of conventional powders, and that the adsorption ability of powders for non-metallic inclusions is still insufficient. It was found that this was due to the fact that the amount was not sufficient. In other words, in the conventional continuous casting of Al-killed steel using powder, a reaction occurs between SiO ₂ in the powder and Al in the molten steel, producing Al ₂ O _3. Therefore, suspended matter such as scum is removed from the powder. Even if the powder can be diluted, if there is too much _SiO2 in the powder, a reaction such as Al + SiO ₂ → Al ₂ O ₃ + Si will occur, which will eventually reduce nonmetallic inclusions such as Al ₂ O ₃ to a certain extent. We found that this is difficult. In addition, even in Si—Mn killed steel with low Al content, Al ₂ O ₃ , CaO—
The adhesion and accumulation of Al ₂ O ₃ etc. caused nozzle blockage, and it was found that the conventional powder was still not able to absorb inclusions sufficiently. From these points, the present inventors changed from the conventional powder composition mainly composed of CaO-SiO ₂ -Al ₂ O ₃ to powder composition mainly composed of CaO-Al ₂ O ₃ , i.e.
As a powder with a composition that reduces the SiO ₂ component as much as possible,
It prevents reactions caused by SiO ₂ in the powder, and mixes appropriate amounts of C, CaF ₂ , Na ₂ O, and MgO to properly adjust physical properties such as viscosity, slagability, and fluidity. This led to the development of a powder with better inclusion absorption performance than conventional powders. Specifically, the powder for coating the hot water surface of the present invention contains more than 40% CaO and less than 60%, Al ₂ O ₃ 20 to 40%,
MgO 0.5% or more and less than 5.0, C0.5~2.0%, _Na2O0.1 ~
5.0%, CaF ₂ 1-10%, and SiO ₂ regulated to 7.0% or less. The powder for coating the continuous casting surface of the present invention will be explained in more detail below. The composition of the powder of this invention is determined as described above, and has a softening temperature of 1200 to 1300°C and a viscosity of 1 to 10 poise (1300°C). The reasons for limiting the composition of this powder are as follows. CaO and Al ₂ O ₃ are the main components of the powder of this invention, with CaO being more than 40% and less than 60%, and Al ₂ O ₃ being 20%.
By blending within the range of ~40%, the non-metallic inclusion adsorption ability, viscosity, softening temperature, and
It can have properties such as basicity. These properties can also be changed by using CaF ₂ , Na ₂ O, etc., but since their contents are smaller than CaO and Al ₂ O ₃ , it is difficult to change them significantly.
Optimal properties are ensured by adjusting the amounts of CaO and Al ₂ O ₃ . Note that if the amount of Al ₂ O ₃ blended is excessive, the ability to adsorb nonmetallic inclusions will deteriorate and the viscosity of the powder will tend to become particularly high (that is, hard), so as mentioned above, it should be kept at 40% or less. MgO, Na ₂ O, and CaF ₂ are all necessary components to finely adjust the powder's properties such as fluidity and slag temperature, but if the amount of MgO added is excessive, the powder may deteriorate. becomes hard, and
On the other hand, if Na ₂ O and CaF ₂ are in excess, they become too soft, making it easier for the refractory bricks to melt or get caught up in the molten steel, and also cause Al in the molten steel and SiO ₂ in the powder to deteriorate. There is a possibility that the reaction with MnO becomes active and the number of Al ₂ O ₃ -based nonmetallic inclusions in the molten steel increases, so it is necessary to set the upper limit of these inclusions as described above. Furthermore, if the amounts added are too small, it will be difficult to properly adjust the characteristics of the powder, so the lower limits of the amounts added are determined as described above. Further, C is an aggregate component for adjusting the powder slag time, softening temperature, etc. If the amount added is excessive, the melting characteristics of the powder will deteriorate, and if it is too small, it will cause excessive melting, which will cause problems. As mentioned above, TC (total carbon content)
Set within the range of 0.5 to 2.0%. On the other hand, it is desirable that SiO ₂ be as small as possible in the powder of the present invention, and by doing so, Al in the molten steel and SiO ₂ in the powder can be reduced as described above.
This can prevent the increase of Al ₂ O ₃ in the molten steel due to the reaction with the molten steel. However, CaO
It may be difficult to maintain properties such as softening temperature with Al ₂ O ₃ alone; for example, if CaO and Al ₂ O ₃
If _the ratio with
In addition to Na ₂ O, it may also be adjusted by adding a small amount (usually 0.2% or more) of SiO ₂ . However, if SiO ₂ exceeds 7%, the aforementioned effect of preventing the increase in Al ₂ O ₃ will hardly be obtained, so even when SiO ₂ is added, the maximum amount must be regulated to 7% or less. As mentioned above, the powder of the present invention suppresses SiO ₂ and contains CaO and Al ₂ O ₃ as main components, and contains small amounts of CaF ₂ and Na ₂ O as fluxes, and small amounts of C and MgO as aggregates. By adding each amount within the above-mentioned range, the melting characteristics and viscosity of the powder can be adjusted appropriately, and the ability of the powder to adsorb non-metallic inclusions, as well as the mold and mold during continuous casting, can be adjusted. This is to sufficiently enhance the lubrication ability between the steel and the steel piece, and to prevent an increase in Al ₂ O ₃ in the molten steel due to the reaction between Al in the molten steel and SiO ₂ in the powder. In addition to the above-mentioned components of the powder, small amounts of Fe ₂ O ₃ , MnO, etc. may be added, but these are only trace components and the basic composition is as described above. The powder of the present invention as described above can be used both for coating the surface of the tundish in continuous casting and for the surface of the mold. However, it is desirable to have slightly different characteristics depending on the location of use. In tundish, the softening temperature and viscosity are set slightly low to emphasize lubricity between the mold inner wall surface and the slab, while in tundish, the viscosity is set slightly high to prevent powder from being drawn in by the amount of molten steel injected from the ladle. It is desirable to do so. The powder of this invention does not solidify even if the amount of Al ₂ O ₃ adsorbed increases, and its viscosity hardly changes.
It also has the feature that the adsorption capacity for Al ₂ O ₃ does not decrease for a long time. Next, an experimental example in which the powder of the present invention was actually used for continuous casting of steel will be described. First, as a preliminary experiment, when the powder of the present invention was used in a continuous casting tandate for manufacturing low-carbon Al-killed cold-rolled steel sheets, bending defects caused by large inclusions were observed when processing the product sheets (cold-rolled steel sheets). was significantly reduced compared to the case of using powder with a conventional composition, and the amount of inclusions deposited in the tundish nozzle was also significantly reduced. However, the specific composition of the powder of this invention used in this experiment is CaO51
%, _{Al2O3 35} %, _SiO2 2.0%, _Na2O2.5 %,
CaF ₂ 3.5%, MgO 3.8%, TC 1.8%. Since the casting conditions for continuous casting in this preliminary experiment were the same as before, the above-mentioned effect was simply due to the composition of the powder, and from this effect, the relationship between Al in the molten steel and SiO ₂ in the powder was determined. It is inferred that almost no Al ₂ O ₃ is produced by the reaction in the powder of the present invention. Next, based on the results of the preliminary experiment described above, Al
This invention can be applied to continuous casting in the production of low-carbon Al-killed cold-rolled steel sheets (Al 0.080%) for deep drawing for cans with a high content, and in continuous casting of SUS430 steel with a low Al content of 0.007 to 0.009%. An experiment was conducted using the powder for both the tundish and the mold, or for either one, and an experiment was also conducted using a conventional powder. However, the powders of this invention have three compositions, powder numbers A ₁ , A ₂ , and A ₃ in Table 1 below, and the conventional powders have the compositions shown in powder number B in Table 1. I used something. In this experiment, for the low carbon Al-killed steel for deep drawing, _the final product, that is, the can obtained by cold rolling and deep drawing, _was
We investigated the occurrence rate of linear defects on the can surface caused by CaO-Al ₂ O ₃ and the occurrence rate of tundice nozzle blockage during continuous casting, and also investigated the oxygen value in the steel for SUS430 steel with Al 0.01% or less. The ability to absorb fine inclusions in molten steel was investigated by measurement. These results are shown in Table 2.

【table】

[Table] However, in Table 2, the linear defect occurrence rate index and the nozzle clogging incidence rate index are the values obtained by indexing each occurrence rate in Experiment No. 6, in which conventional powder was used for the tundish and mold, as 100.
In addition, the oxygen value in SUS430 steel is the average value of 100 cases. In addition, in Table 2, experiment numbers 1 to 3 are examples in which powders A ₁ to _{A 3 of} the present invention were used for both _the tundish and the mold, respectively. _SiO2 as shown respectively
They have different contents. On the other hand, in Experiment No. 4, the powder A ₁ of the present invention was used only for the tundish, and in Experiment No. 5, the powder A ₁ of the present invention was used only for the mold. From the results shown in Table 2, if the powder of this invention is used, Al ₂ O ₃ clusters or CaO-Al ₂ O ₃
It is clear that linear defects caused by large inclusions in the system are reduced, and this effect becomes more pronounced as the SiO ₂ content of the powder decreases. If it is used for only one of them, it will be more noticeable. On the other hand, the nozzle blockage occurrence rate also follows a similar trend. However, since the nozzle in this case is between the tundish and the mold, the effect of reducing the nozzle clogging rate cannot be obtained even if the powder of the present invention is used only in the mold. Also, Al content is 0.007~0.009%
By using the powder of this invention, the oxygen value in SUS430 steel, which is in the range of is clear. As is clear from the above explanation, the powder for coating the continuous casting surface of the present invention prevents the Al ₂ O ₃ in the molten steel from increasing due to the reaction between Al in the molten steel and SiO ₂ etc. in the powder. Therefore, when used in a tundish, clogging of the tundish nozzle due to adhesion and accumulation of Al ₂ O ₃ can be prevented as much as possible, and as mentioned above, an increase in Al ₂ O ₃ can be prevented. This, combined with the fact that it has a much higher ability to absorb other fine inclusions than conventional powders, means that when used in both the tundish and/or mold, the cleanliness of the slab will be much better than conventional powders. Various effects can be obtained, such as reducing the defect rate of products and reducing the amount of oxygen in the steel.

Claims (1)

[Claims] 1 CaO more than 40% and less than 60%, Al ₂ O ₃ 20-40%,
MgO0.5% to less than 5.0%, C0.5 to 2.0%,
Contains 0.1-5.0% Na ₂ O, 1-10% CaF ₂ and SiO ₂
Powder for coating the surface of hot water in a tundish or mold in continuous casting of steel, characterized in that the amount of water is regulated to 7.0% or less.