JPS6353902B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a powder for continuous casting, and in particular, an object of the present invention is to provide a powder for continuous casting that can realize smooth casting operations even in high-speed casting where the drawing speed of slabs is high. be.

The so-called continuous casting method, which continuously manufactures slabs, blooms, billets, and other slabs from molten steel, is rapidly becoming more popular due to its advantages in terms of yield, resource conservation, and energy conservation. Furthermore, in recent years, there has been a trend to increase casting speeds with the aim of improving productivity, and for example, in continuous slab casting, high-speed castings such as drawing speeds of 1.4 to 2.0 m/min are being implemented.

However, in high-speed casting as described above, the thickness of the solidified shell at the mold outlet is relatively thin compared to the case of casting at a normal drawing speed of around 1.0 m/min. The probability of a spill accident of unsolidified molten steel will increase. Especially when the length of the mold is 700~
When using an ordinary mold of about 900 mm, there is a strong possibility that breakout will occur if the drawing speed exceeds 1.4 m/min.

Not only is this breakout extremely dangerous, but once it occurs, it takes a long time to recover, and the cost involved is enormous.

In addition, in continuous casting of steel slabs for thick plates and pipes that generally contain about 0.6 to 1.7% by weight of Mn, wide-sided slabs are particularly likely to occur when the C concentration in the steel is in the range of 0.09 to 0.15% by weight. The tendency of vertical cracks to occur becomes more pronounced as the casting speed increases.

When these vertical cracks occur, care is required to remove them prior to rolling, which not only causes a decrease in yield, but also hinders the application of direct heating and direct rolling of heated pieces. In severe cases, it may not be possible to use the product as a product.

Therefore, in increasing the speed of continuous casting, there is an urgent need to solve operational problems such as breakouts and quality problems such as vertical cracks.

Incidentally, the occurrence of breakouts and vertical cracks as described above is particularly closely related to the physical properties of the powder added to the surface of the mold in the mold during continuous casting (hereinafter simply referred to as powder for continuous casting), as described below.

That is, breakout often occurs when the amount of the molten powder layer flowing from the meniscus in the mold into the space between the inner wall of the mold and the slab is reduced, and lubrication there is not performed smoothly.

Also, vertical cracking occurs, especially when the C concentration in the steel is 0.09 to 0.15.
Vertical cracks on the broad surface of the slab that occur frequently when the weight percentage is
It is thought that it is generated by the following mechanism.

Generally, when continuous casting powders of the same composition are used, as the drawing speed increases, the amount of the molten powder layer flowing from the meniscus per unit weight of molten steel to be cast decreases. On the other hand, since the amount of molten steel injected per unit time increases, the molten steel flow velocity and temperature distribution near the molten metal surface in the mold tend to become non-uniform in the width direction. Therefore, the amount of powder flowing into the molten layer from the meniscus tends to increase at the narrow surface and decrease at the center of the wide surface near the immersion nozzle. Therefore, at the center of the wide surface, the thickness of the slag film that forms on the surface of the slab becomes thinner and is subjected to supercooling during initial solidification, resulting in more "wrinkles" on the surface of the slab than in other areas. Depression marks are likely to occur. This leads to non-uniform solidification, and the slag, which has relatively high fluidity immediately after it flows from the meniscus, tends to concentrate at the depression mark part where the flow rate is relatively high, resulting in non-uniform inflow. . When this uneven inflow develops, a depression called a slag streak is formed. In addition, a fixed layer in a semi-molten state called slag rim is formed along the inner circumference of the mold in the meniscus area, but excessive development of this slag rim causes an imbalance in the widthwise distribution of the inflow of the molten powder layer from the meniscus area. This causes uniformity.

Furthermore, the non-uniform inflow of the molten powder layer at the center of the wide surface locally increases the thickness of the slag film, increasing the frictional stress when the slab shrinks in the width direction due to solidification shrinkage.

As shown above, when the C concentration is 0.09 to 0.15%, the amount of solidification shrinkage becomes particularly large, the non-uniformity of the solidified shell thickness due to the formation of depression marks due to overcooling of the meniscus, and the frictional stress between the mold inner wall surface and the slab surface. It is thought that thermal stress and edge bending stress, which are maximum at the center of the wide surface, act non-uniformly in the width direction, leading to wide vertical cracks.

From the above, it can be seen that breakouts and vertical cracks are likely to occur when the amount of the molten powder layer flowing out from the meniscus portion is too small or when the slag rim develops excessively as described above. Therefore, in order to perform high-speed casting without causing the above-mentioned problems, it is important to ensure that the inflow amount of the molten powder layer is commensurate with the drawing speed so that the formation of slag rim is small and mild.

Therefore, the inventors decided to solve the above-mentioned problem by improving powder for continuous casting, and conducted detailed research on its physical properties. As a result, the physical properties of powder for continuous casting are defined as the viscosity when the powder is melted, the activation energy indicating its temperature dependence, the difference between the softening melting temperature and the solidification temperature, and the arithmetic mean value of the softening melting temperature and the solidification temperature. By discovering that the characteristic temperature of the powder is particularly important, and controlling these physical properties within appropriate ranges, we can almost completely prevent breakouts, which are a concern during high-speed casting operations, and The occurrence of vertical cracks on wide slab surfaces was also significantly reduced.

That is, this invention provides 39 to 46% CaO by weight,
Contains 40-56% _SiO2 and 2-15% _Al2O3 as main components, oxides of alkali metals _such as _Na2O , _K2O , _Li2O , _CaF2 , _BaF2 , NaF, Alkaline earth metals or alkali metal fluorides such as LiF, Na ₂ CO ₃ , K ₂ CO ₃ , Li ₂ CO ₃ , CaCO ₃ , BaCO ₃
Continuous casting powder containing at least one subcomponent selected from carbonates of alkali metals or alkaline earth metals such as ) The activation energy E of viscosity expressed by the formula is
It is a powder for continuous casting which has a temperature of 20 Kcal/mol or less, a difference between a softening melting temperature and a solidification temperature of 120 degrees or less, and a characteristic temperature expressed as the arithmetic average of them in a range of 850°C or more and 1150°C or less.

E=RT log (η/η ⁰ ) (cal/mol) …(1) Here, η is the viscosity (poise) at temperature T (K),
η ⁰ is a frequency factor related to viscosity, and R is a gas constant (1.986 cal/deg・mol). This is derived from the results of an investigation into the occurrence of breakouts and wide vertical cracks in slabs by adding continuous casting powders with various physical properties. The viscosity η, the viscosity activation energy E, and the characteristic temperature θ _c were determined by the methods described below.

(a) Method for measuring viscosity η when melting powder for continuous casting; Hold in an atmospheric furnace at 900°C for about 4 hours to burn out and remove carbon powder contained in the powder for continuous casting in advance. Weigh out 200 to 250 g of continuous casting powder from which carbon powder has been removed in a platinum crucible, hold it at 1300 to 1400°C for about 1 hour, and
The temperature is maintained at 1400°C for 30 minutes to 1 hour to uniformly melt the powder for continuous casting and to degas it at the same time. Next, platinum balls are immersed in uniformly melted continuous casting powder, and heated from 1400℃ using the platinum ball pulling method.
The viscosity was measured sequentially at 50°C intervals toward the lower temperature side. Each measurement temperature was held for approximately 30 minutes to ensure uniform temperature.

(b) Method for calculating viscosity activation energy E; Measure the viscosity of the powder for continuous casting when melting using the method described in (a) above at intervals of 50°C from 1400°C to a temperature at which measurement is no longer possible. Regarding the temperature dependence of viscosity, activation energy E was calculated assuming an Arrhenius type as shown in the following equation (1).

η=η ⁰ exp (E/RT) ∴E=RT log (η/η ⁰ ) (cal/mol) …(1) Here, η is the viscosity at temperature T (K), and η ⁰ is the frequency factor related to viscosity. , R is the gas constant (cal/
mol・deg).

(c) How to determine the characteristic temperature θ _c ; Weigh out approximately 0.5 g of continuous casting powder from which carbon powder has been removed in advance by the method explained in (a) above in an alumina boat, and place it in a furnace at a predetermined temperature. After holding it for 15 minutes, take it out of the furnace. Observe the melting state of continuous casting powder on the board, and define the softening temperature θ _np as the temperature when no unmelted powder remains and the surface is smoothly melted. In the viscosity measurement method, viscosity is measured at 30°C intervals near the temperature at which the solid phase begins to crystallize, and the temperature at which viscosity cannot be measured in the platinum ball pulling method is defined as the solidification temperature θ _fp . did. The arithmetic mean value θ _c =(θ _np + θ _fp )/2 was defined as the characteristic temperature θ _c .

In other words, as is well known, powder for continuous casting is a mixture of various melting point components, so it not only does not have a specific melting point like a single compound, but also generally behaves differently when heated and cooled. Therefore, θ _c is also taken as a representative value of the melting and solidification temperature of the powder.

An example of the composition of the continuous casting powder used in the above experiment is as follows.

CaO35%, _SiO2 44%, _{Al2O3 2} %, _Na2O10 %,
F9%, viscosity η, activation energy E and characteristic temperature _θc were changed as follows.

(1) Viscosity η: The viscosity was adjusted by changing the CaO/SiO ₂ ratio and the amount of Al ₂ O ₃ as well as the type and content of fluoride.

(2) Activation energy E: In Figure 1, using NaF as a viscosity modifier,
The relationship between activation energy E and CaO/SiO ₂ when Al ₂ O ₃ in the powder for continuous casting is 4% is shown.

(3) Characteristic temperature θ _c : All or some percentage of the fluoride used as a viscosity modifier is mixed with the base and melted in advance. Thereafter, this is cooled, crushed, and used as a raw material for continuous casting powder. By this method, the softening and melting temperature of the powder for continuous casting is adjusted. Furthermore, the main components are almost the same, FeO,
Use materials with slightly different components such as MnO and MgO.

In the above experiments, when powder for continuous casting whose composition and physical properties satisfy the appropriate range of the present invention was used, no breakout or longitudinal cracking occurred even in high-speed casting with a drawing speed of 1.4 m/min. Smooth operations were achieved.

The reason why the components of the powder are limited to the above ranges in this invention is as follows.

If the continuous casting powder contains less CaO and more _SiO2 ,
The absorption of Al ₂ O ₃ non-metallic inclusions floating from the molten steel in the mold becomes poor, leading to slag on the slab surface and accumulation of Al ₂ O ₃ non-metallic inclusions just below the surface. If there is too much SiO ₂ , Al ₂ O ₃
Although the absorption capacity of system inclusions increases, it becomes difficult to bring θ _c and η within the desired range.
CaO was limited to a range of 39-46%, and _SiO2 was limited to a range of 40-56%. Also, if Al ₂ O ₃ is less than 2%, the softening and melting temperature becomes too high, while if it exceeds 15%, anorthite (CaO・SiO ₂・2Al ₂ O ₃ ) and gehlenite (2CaO・SiO ₂・Al ₂ O ₃ ) Not only does it tend to crystallize and increase slag, but also θ _c and η become too high, so it was limited to a range of 2 to 15%.

Next, the reason why the physical properties of the powder for continuous casting were limited to the above range will be described.

If the viscosity η is less than 1.0 poise, even if other physical properties are within appropriate ranges, the C concentration in the steel will be particularly low.
The effect of reducing longitudinal cracks in slabs containing 0.09 to 0.15 wt% becomes poor, while when exceeding 6.0 poise,
Insufficient inflow of the slab molten layer from the meniscus,
Lubrication will be poor.

If the activation energy E is small, the temperature dependence of viscosity is low, so even if the temperature distribution in the circumferential direction of the meniscus in the mold changes slightly, it is difficult to cause unevenness in the amount of slag inflow, and therefore smooth casting is possible. Yes, but
Even if a powder with an activation energy exceeding 20 Kcal/mol has an appropriate viscosity and characteristic temperature of 1300°C, the viscosity has a large temperature dependence, so it is unsuitable for high-speed casting, and it is not suitable for rapid changes in drawing speed or abnormalities. Uneven or insufficient flow of slag occurs during continuous casting of steel types, width changes, etc. Therefore, the activation energy is less than 20 Kcal/mol, preferably 15 Kcal/mol.
By doing so, a sufficient improvement effect can be obtained compared to the conventional method, and especially when the setting is 15 Kcal/mol or less, breakout and C concentration can be reduced.
At 0.09 to 0.15% by weight, wide vertical cracks, which are particularly likely to occur, were significantly reduced, and a remarkable improvement effect was observed.

The reason why the difference between the softening temperature and the solidification temperature is set to within 120 degrees is as follows.

The difference between the softening temperature and the solidification temperature has a decisive effect on the state of slag rim formation in the meniscus portion of the mold. When this temperature difference exceeds 120 degrees, the slag rim develops to an extreme degree, and as a result, the inflow of molten slag from the meniscus becomes uneven, leading to an increase in vertical cracks. In other words, it is desirable to reduce this temperature difference, and in order to reduce vertical cracking, 120
It is necessary to keep the amount within a certain degree, and the development of slag rim can be made mild. More preferably, the angle is within 80 degrees.By doing this, the slag rim is made extremely light, and as a result, the inflow of slag from the meniscus is made uniform, and the formation of slag streaks can be prevented. Vertical cracking in the range can be advantageously prevented.

The lower the value of the characteristic temperature θ _c of the powder, the
The thickness of the molten powder layer on the surface of the mold becomes thicker, and the development of slag rim is slight, but if it is lower than 850℃, it will adhere to the roll at the mold outlet, making it difficult to cast for a long time. If the temperature exceeds ℃, the expected thickness of the molten layer at the surface of the molten metal will not be obtained, and the amount of slag inflow from the meniscus will be insufficient, increasing the frequency of occurrence of breakouts and wide vertical cracks. Therefore, the appropriate range for the characteristic temperature θ _c is 850 to 1150°C.
And so.

Although the application of the powder according to the present invention to continuous slab casting has been mainly explained above, the powder according to the present invention is not limited to this, and can also be applied to high-speed continuous casting of blooms because it promotes lubrication in the mold and prevents breakout. It was confirmed that this method is extremely effective, similar to continuous slab casting. Furthermore, when the powder for continuous casting according to the present invention is used, the thickness of the molten layer at the surface of the molten steel in the mold becomes sufficiently thick, which not only promotes the dissolution of inclusions such as alumina that float from the molten steel in the mold during casting, but also improves the melting of slag. Since the formation of these particles becomes light, the occurrence of slag caused by these particles biting into the surface of the slab is also reduced. Furthermore, when the drawing speed is suddenly changed from low to high speed,
In some cases, the flow of slag from the meniscus part could not be followed, resulting in large vertical cracks, but the powder for continuous casting according to the present invention can maintain a sufficiently thick molten layer at the surface of the molten metal, as described above. The inflow of slag is ensured even during such unsteady conditions, lubrication within the mold is promoted, and there is no risk of surface cracking.

[Brief explanation of the drawing]

Figure 1 shows the results when the continuous casting powder contains 4% Al ₂ O ₃ and NaF is used as the viscosity modifier.
It is a graph showing the relationship between CaO/SiO ₂ ratio and activation energy E.

Claims (1)

[Claims] 1 Contains as main components 39-46% CaO, 40-56% SiO ₂ and 2-15% Al ₂ O ₃ by weight,
Alkali metal oxides such as Na ₂ O, K ₂ O, Li ₂ O, etc.
Alkaline earth metals or alkali metal fluorides such as CaF ₂ , BaF ₂ , NaF, LiF, and Na ₂ CO ₃ ,
A powder for continuous casting containing as a subcomponent at least one selected from carbonates of alkali metals or alkaline earth metals such as K ₂ CO ₃ , Li ₂ CO ₃ , CaCO ₃ , BaCO ₃ , etc. The viscosity is 1.0 poise or more and 6.0 poise or less, the activation energy E of the viscosity expressed by the following equation (1) is 20 Kcal/mol or less, and the difference between the softening melting temperature and solidification temperature is 120
Powder for continuous casting characterized by having a characteristic temperature within the range of 850°C or more and 1150°C or less, expressed as the arithmetic mean of these temperatures. E=RT log (η/η ⁰ ) (cal/mol) …(1) Here, η is the viscosity (poise) at temperature T (K),
η ⁰ is the frequency factor related to viscosity, R is the gas constant (1.986 cal/deg・mol)