JP2001205405A - Initial solidification control method for steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an initial solidification control method for steel which is applicable even to broad casting mold shapes, casting mold sizes and steel kinds. SOLUTION: The required average taper TP1 (%/m) of an area in contact with a solidified shell during the time up to 3 seconds after the solidification start in a meniscus position of the casting mold which is set at the prescribed outlet dimensions meeting the sectional shape of the slab and is imparted with such a taper as to increase the distance to the opposite inner walls from an outlet to an inlet is specified to the value expressed by the following equation with respect to a solidification shrinkage index K and a casting mold sectional shape index L1.Log10 (L2/L1). The required average taper TP2 (%/m) of the area in contact with the solidified shell after 3 seconds after the solidification start is specified to 0.6 to 1.3%/m. The difference between the distance between the inner walls of the casting mold when the required taper TP1 and the required average taper TP2 are imparted thereto and the actual distance between the inner walls of the casting mold is confined within 1.5×10-3m. TP1 = j.[(2.1+0.8K)/ 17.L1.Log10(L2/L1)+1.0}], where j: coefficient (j = 0.5 to 1.5). Accordingly, the surface defect of the slab and the burn of the slab may be effectively suppressed and the improvement in quality and stable operation are made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling the initial solidification of steel, which can prevent surface defects of a slab as much as possible in continuous casting of ordinary steel, regardless of the type of steel or mold. It is about the method.

[0002]

2. Description of the Related Art In continuous casting of steel, the initial solidification conditions are controlled according to the difference in casting temperature and solidification shrinkage state for each steel type or the bulging state of the solidified shell in the mold for each mold shape. It is important to prevent surface defects of the slab.

Conventionally, a number of inventions have been made with respect to techniques for optimizing casting conditions on the premise of each local condition. For example, JP-A-2-284
No. 747 discloses a technique for optimizing a mold taper for a medium carbon steel slab, and Japanese Patent Laid-Open No. 7-9104 discloses a technique for optimizing the physical properties and oscillation conditions of a mold powder for the purpose of uniform inflow of the mold powder. Japanese Patent Application Laid-Open No. 7-60422 discloses a technique for optimizing the consumption of mold powder, mold taper, and casting speed in order to prevent corner cracking of Nb-containing ultra-low carbon steel.
Japanese Patent Publication No. 9-62014 discloses a technique for reducing surface cracks by optimizing the viscosity and oscillation conditions of mold powder for medium carbon steel slabs.
No. 3 discloses a technique for controlling the oscillation conditions of a mold to prevent vertical cracks in a slab.

[0004]

However, each of these techniques merely discloses an improvement example on the premise of a specific local condition (mold shape / size, steel type). It does not provide a universal initial pseudo-solid condition applicable to steel grades.

[0005] For example, the invention disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 7-60422 not only corresponds to a specific steel type, but also the oscillation conditions proposed in the present invention depend on the mold sectional shape. Since no consideration is given to the effect of the method, the method cannot be applied to a continuous casting machine having a special mold section such as a round section.

That is, the above-mentioned improvement example comprehensively considers the influence of the steel type and the shape of the mold while considering the three conditions indispensable for the initial solidification control: mold powder, oscillation, and mold taper. Therefore, any steel type or mold shape was not adequate for the purpose of effectively preventing the surface defects of the slab.

For example, even if the mold taper disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2-284747 is applied, if the mold powder or the oscillation conditions are inappropriate, a vertical crack in the slab or casting in the mold may occur. The inventor has confirmed through experiments that the problem such as burning of pieces frequently occurs.

Further, even if the consumption of the mold powder and the mold taper disclosed in Japanese Patent Application Laid-Open No. 7-60422 are controlled, if the oscillation conditions are inappropriate, corner cracking of the Nb-containing ultra-low carbon steel occurs. .

Further, Japanese Patent Application Laid-Open No. 7-9104, Japanese Patent Publication No.
The same can be said for JP-62014 and JP-B-61-60703.

[0010] As described above, if the conventional technique is applied when the conditions such as the mold shape and the steel type change, there is no effect.
In some cases, it often resulted in operational and quality problems. Or, if other unspecified conditions change, in addition to ensuring quality,
The disclosed technical elements were technically inadequate, such as making operation even more difficult.

The present invention has been made in view of the above-mentioned problems in the prior art, and provides a universal initial solidification control method for steel that can be applied to a wide range of mold shapes, mold sizes, and steel types. It is intended to provide.

[0012]

In order to achieve the above object, the present invention provides a mold sectional shape index L ₁ · Log ₁₀ (L ₂₎ indicating the degree of adhesion between a solidified shell and a mold in the mold.
/ L ₁ ) and the solidification shrinkage index K, which is an index simply representing the amount of shrinkage of the solidified shell in the actual mold due to the δ → γ transformation, are three technical elements indispensable for initial solidification control. Universal initial solidification conditions for mold powder, oscillation, and mold taper are to be obtained. In this way, in controlling the initial solidification of steel, it can be universally applied to a wide range of mold shapes, mold sizes, and steel types.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In order to overcome the limitations of the prior art as described above, the present inventor repeatedly performed casting experiments on a wide range of mold shapes, mold sizes, and steel types. ₁ · Log ₁₀ (L ₂ / L ₁ )
And solidification shrinkage index K, devised as an index that simply represents solidification shrinkage due to the mold cross-sectional shape and δ → γ transformation that greatly affects the initial solidification of steel, using these indices,
The universal initial solidification conditions were derived without fail for three technical elements that are indispensable for initial solidification control, namely, mold powder, oscillation, and mold taper, and the following invention was established. Hereafter, mold powder
The description will be made in the order of the oscillation and the mold taper.

A. Mold powder of the general composition containing F The composition and physical properties CaO · SiO ₂ in the mold powder as the main component, Kasupidin as one of the main crystal composition during solidification (3CaO · 2SiO ₂ · CaF ₂
) Is known to precipitate.

Regarding the precipitation of cuspidin, recently, C
AMP-ISIJ Vol. 11 (1998), P18
6, CAMP-ISIJ Vol. 12 (199
9), there are reports such as P788. As described in these documents, the precipitation of cuspidin is a phenomenon that greatly affects the thermal conductivity of the powder film existing between the mold and the solidified shell.

In a mold powder having a general composition containing CaO.SiO ₂ as a main component and F, a powder film having a basicity in the range of about 0.7 to 1.4, which is generally applied, is accompanied by an increase in basicity. It is known that crystallization (precipitation of cuspidin) is promoted. This is about 1.9 when the basicity of the cuspidin composition is calculated as a binary system of CaO—SiO ₂ (Ca in CaF ₂ is converted to CaO), and it is about 1.9. (Here, only the main components CaO and SiO ₂ are considered,
The effects of MgO, Al ₂ O ₃ and TiO ₂ are neglected. ).

The crystallization of the powder film existing in the gap between the mold and the solidified shell is carried out, for example, by using iron and steel Vol. 82
(1996) No. 6, P504-508, iron and steel V
ol. 83 (1997) No. 2, pp. 115-120, is thought to affect the radiant heat conduction of the powder film and the thermal resistance between the powder film and the mold interface, and greatly controls the heat flux (heat removal behavior) in the mold. Is a factor.

As an example of a technique for controlling the heat flux in the mold by the mold powder, as a measure against longitudinal cracking of medium carbon steel (subperitectic steel), a mold powder having a high basicity and a high solidification temperature is applied to reduce the heat flux in the mold. Numerous reports have been made on the effectiveness of techniques for reducing the temperature (slow cooling technique in the mold) (eg, CAMP)
-ISIJ Vol. 3 (1990), P1226, C
AMP-ISIJ Vol. 4 (1991), P124
7, CAMP-ISIJVol. 5 (1992), P1
221).

However, since the mold is a container for cooling and solidifying the molten steel, it is not sufficient to simply cool the molten steel. It is anticipated that too slow slow cooling will cause adverse effects due to impairment of the original cooling function of the mold.

Based on such prediction, the present inventor has conducted research from the viewpoint of appropriate slow cooling. As a result, a “mold cross-sectional shape index” indicating the degree of adhesion between the solidified shell and the mold in the mold was devised, and the appropriate basicity of the mold powder was calculated using the “mold cross-sectional shape index” by the following equation (1).
As shown in FIG.

[0021]

## EQU1 ## (CaO + 1.2MgO) / (SiO ₂ +0.
5Al ₂ O ₃ + 0.7TiO ₂ ) = a · [1.0 · {L ₁ · Log ₁₀ (L ₂ / L ₁ )} ^0.3
+1.0] where L _{1 is} the shortest distance from the center point of the mold section to the inner wall of the mold (m) L _{2 is} the longest distance from the center point of the mold section to the inner wall of the mold (m) a: Coefficient (a = 0.6 to 1.1)

L ₁ · Log ₁₀ (L ₂
/ L ₁ ) is a mold sectional shape index devised by the present inventors. The mold sectional shape index is, for example, 0 in the case of a round sectional mold, and 0. 0 in the case of a square sectional mold having a side of 0.15 m.
It is designed to take a large value in the case of a slab continuous casting machine mold having a cross section of 0113, 0.21 × 1.85 m and a large value in the case of a cross section having a large bulging of a solidified shell.

Since bulging is a phenomenon in which a solidified shell expands due to static pressure of molten steel, it can be said that bulging is a deformation that approaches a circular cross section that is the strongest (hardly deformable) shape against internal pressure. "Cross-section shape index" indicates that the rigidity of the solidified shell increases from the circular cross-section to a regular polygon, square, or rectangle, as the cross-section becomes weaker (easy to deform) against internal pressure, or the length of one side becomes longer. It is an index of the ease of bulging, in which the lower the value, the larger the value.

It is considered that bulging of the solidified shell in the mold has an effect of increasing the adhesion between the mold and the solidified shell. Therefore, the present inventor thought that the devised mold cross-sectional shape index could be an index indicating the degree of adhesion between the mold and the solidified shell.

On the other hand, the present inventor has conducted a detailed investigation on the structure of the powder film. As a result, the powder film has a small vertical vibration called oscillation in the gap between the mold and the solidified shell, but in the long term, Between the stationary mold and the solidified shell descending at the casting speed, it descended while being subjected to shear deformation, and traces were observed that seemed to have filled the gap between the lower mold and the solidified shell.

Such a shear deformation hardly occurs in a crystal layer which is a completely solid powder powder, but mainly occurs in a glass layer. That is,
It is said that the powder film in which crystallization has progressed is less likely to be filled in the gap between the lower mold and the solidified shell while undergoing shear deformation, and that the filling property in the gap between the mold and the solidified shell is poor.

The present inventor has set forth the above-mentioned “mold sectional shape index”.
And the inference obtained from the survey results of the powder film were combined to obtain a mold sectional shape index L ₁ · Log ₁₀
As a result of investigating the relationship between (L ₂ / L ₁ ) and proper crystallization of the powder film (basicity of the powder), the result indicated by the above-mentioned formula 1 was reached.

In the above equation (1), the smaller the cross-sectional shape index of the mold, that is, the lower the degree of adhesion between the mold and the solidified shell, the better the filling of the gap between the mold and the solidified shell (the crystallization is suppressed). ) Means that it should be cast in combination with mold powder. Conversely, the mold cross-sectional shape index is large, that is, when the adhesion between the mold and the solidified shell is good, there is little adverse effect that the filling property to the gap between the mold and the solidified shell is poor, rather,
Considering that good adhesion between the mold and the solidified shell acts in the direction of increasing the heat flux in the mold by reducing the gap between the mold and the solidified shell, in such a case, the slow cooling effect is high. (High crystallization) means that it should be cast in combination with mold powder.

It should be noted that Mg on the left side of the above equation 1
The inventor of the present invention has experimentally determined the effect of each of the coefficients given to O, Al ₂ O ₃ , and TiO ₂ on crystallization.

The reasons for limiting other compositions are as follows.
It is. I_A Group metal oxide: I_A Group metal oxides as carbonates
When melted, including SiO2 _Two React with
Generates a liquid phase with a low melting point, which helps mold powder to melt uniformly.
Cause obstacles. Therefore, in the present invention, it is set to 10% or less.

F: F is an element effective for stably depositing cuspidin. According to various experiments by the present inventors, 1.5% or more was required. ZrO ₂ : When ZrO ₂ is contained in a large amount, it becomes a crystal nucleus due to its high melting point, and the crystallization of the powder film becomes excessive.
Therefore, in the present invention, the content is set to 5% or less.

MnO: MnO has the effect of suppressing crystallization of the powder film, and is also easily reduced and decomposed into Mn and O, thus causing a change in the physical properties of the mold powder. Therefore, in the present invention, it is limited to 5% or less.

B ₂ O ₃ : B ₂ O ₃ has a great effect of suppressing the crystallization of the powder film, and also has
Has the effect of making the grain boundaries brittle. Therefore,
In the present invention, it is limited to 5% or less.

Further, the present inventor, after satisfying the above-mentioned mold powder composition and further controlling the solidification point Sp which satisfies the following equation 2 according to the type of steel ([C] range), obtains a surface such as a vertical crack. It has been experimentally found that both the number of scratches and the problem of seizure of the solidified shell on the mold are reduced.

[0035]

## EQU2 ## Sp = T _SL −b + c ^K + d · Log ₁₀ (0.0
6 · Vc) where T _SL : solidus temperature (K) obtained from the equilibrium diagram of steel K: solidification shrinkage index [= α / {T _LS − (T _δγ −5)}] α: fully solidified _Tδγ : Temperature (K) at the midpoint between the start point and end point of the transformation from δ phase to γ phase determined from the equilibrium diagram of steel Vc: Casting speed (× 10 ^-3 m / sec) b: Coefficient (b = 300 to 400) c: Coefficient (c = 3.0 to 3.5) d: Coefficient (d = 40 to 50)

The solidification shrinkage index K used for obtaining the solidification point Sp of the mold powder described above was devised by the present inventor as an index that simply represents the shrinkage amount of the solidified shell in the actual mold due to the δ → γ transformation. is there.

Α used to obtain the solidification contraction index K is the volume ratio (%) of the δ phase at the time of complete solidification (solid phase ratio: fs = 1.0). Since the δ → γ transformation amount of the solidified shell having the later strength and the accompanying shrinkage increase, the solidified shell as a whole overcomes the static pressure of molten steel and the deformation resistance of the surrounding solidified shell that has already solidified. It is a physical fact to try to cause a large contraction.

Further, T _SL − in the equation for obtaining the coagulation contraction index K is
T _{[Delta] [gamma]} term is the difference between the temperature T _{[Delta] [gamma]} of the middle point between the start and end points of the equilibrium diagram than obtained [delta] → gamma transformation of the steel. This difference is an index indicating how far the completely solidified point of the solidified shell is from the point where the δ → γ transformation occurs.The smaller the difference, the thinner the solidified shell and the smaller the molten steel static pressure, that is, the shrinkage deformation At the point when the resistance to
→ Since γ transformation occurs, an actual contraction amount close to the amount to be contracted can be obtained.

The coagulation shrinkage index K can be calculated from both (α and T _SL
−T _δγ ), that is, the basic idea is that it is the product of the amount to be coagulated and contracted by the δ → γ transformation and its feasibility. The number “5” in the equation for determining the coagulation contraction index K is an adjustment coefficient.

The present inventor has repeated the actual casting test, and has obtained an empirical formula capable of giving an appropriate powder solidification point Sp using the solidification shrinkage index K devised as described above,
That is, Equation 2 described above was derived.

In Equation 2, c ^K involving coagulation contraction index ^K
The term is that the larger the solidification shrinkage accompanying the δ → γ transformation, the more appropriately the solidification point of the mold powder is increased, the proportion of the solid phase which is a crystal precipitation site in the powder film is increased, and the cooling effect in the mold by the precipitation of cuspidin is increased. This is a term that quantitatively expresses the technique of promoting In addition, the T _SL term expresses a method of setting the solidification point of the mold powder in accordance with the temperature at which the solidified shell starts to have strength, that is, T _SL , from the viewpoint of ensuring lubricity (the solidified shell is not broken). . Conventionally, from the viewpoint of determining the solidification point of powder according to the absolute temperature of molten steel, it is possible to determine the powder solidification point according to the liquidus temperature (T _LL ) of the steel,
→ There is an idea that the powder solidification point is determined according to the γ transformation end temperature (T _γ ), but from the viewpoint of ensuring lubricity that the solidified shell is not broken, the solidus temperature (T _SL )
The freezing point of the powder should be determined accordingly. In addition, d · Log ₁₀ (0.06 ·
The term Vc) corresponds to the solidification shell surface temperature that changes according to the casting speed Vc, and expresses a method of setting the solidification temperature. In addition, b, c, d and the number "0.06"
Is an adjustment coefficient.

That is, the initial hardening of the steel according to the first present invention.
The solid control method is CaO.S in continuous casting of ordinary steel.
iO_Two Contains F as the main component and the main crystal group during solidification
Caspidin (3CaO.2SiO)_Two 
・ CaF_Two ) Of the mold powder, CaO,
MgO, SiO_Two , Al_Two O_Three , TiO_Two Mass concentration ratio of
Basicity expressed by: ｛(CaO + 1.2MgO) / (SiO
_Two + 0.5Al_Two O_Three +0.7 TiO _Two )｝, But mold cut
Surface shape index L₁ ・ Log_Ten(L_Two / L₁ ) Against
It is expressed by Equation 1 described above, and expressed by the mass concentration of other components.
Composition is I_A Group metal oxide ≦ 10%, F ≧ 1.5%,
ZrO_Two ≦ 5%, MnO ≦ 5%, B_Two O_Three ≤5%
Furthermore, the solidification point Sp (K) is calculated from the equilibrium diagram of steel.
The calculated solidus temperature T_LS(K) and the above-mentioned coagulation shrinkage index
K and casting speed Vc (× 10^-3m / sec)
Having a composition and physical properties represented by Formula 2
The point is to use powder.

B. Template Oscillation Conditions Template oscillation was invented in the 1930s and
It was widely known that the development of a vibration method having a negative strip (making the mold descending speed faster than the casting speed) stage in the 960's played an important role in the practical use of continuous casting of steel. Have been.

The success of the negative strip illustrates the effectiveness of the mold descending faster than the slab. In other words, it is important that "the distance over which the cast slab is to be stroked by overtaking the slab" in the entire descending stroke of the mold. Then, assuming that the distance over which the slab is overtaken and the stroke is taken is the effective stroke Se, the effective stroke Se is such that the vibration is s as shown in FIG.
In the case of an in-waveform, it can be mathematically obtained as in the following Expression 3. In FIG. 1A, the vertical axis y indicates amplitude, and the horizontal axis t indicates time. FIG. 1B is obtained by differentiating the vertical axis of FIG. 1A, and the area of the hatched portion represents the effective stroke Se.

[0045]

Equation 3] _{Se = (U N -Vc) ·} T N However, U _N: Average template lowering speed during negative strip (× 10 ^-3 m / sec) = S / T _N · sin [cos ^-1 {- Vc / (π · S ·
f)｝] S: Oscillation total stroke (× 10 ⁻³ m) T _N : Negative strip time (sec) = 1 / (π · f) · cos ⁻¹ {Vc / (π · S · f)} f : Oscillation frequency (Hz)

In the case where the vibration has a non-sin waveform as shown in FIG.
An approximate value of the effective stroke Se can be easily obtained by using the following Expression 4 obtained by approximating the waveform. Since the frequency fd of the descending waveform used for obtaining the following equation 4 is fd = f at the time of the sin waveform, the following equation 4 can be applied to both the sin waveform and the non-sin waveform. .

[0047]

Equation 4] _{Se = (U N -Vc) ·} T N However, U _N: Average template lowering speed during negative strip (× 10 ^-3 m / sec) = S / T _N · sin [cos ^-1 {- Vc / (π · S · fd
)｝] S: Oscillation full stroke (× 10 ⁻³ m) T _N : Negative strip time (sec) = 1 / (π · fd) · cos ⁻¹ ｛Vc / (π · S · fd)
)｝ Fd: frequency of descending waveform = f / (1−λ / 100) f: oscillation frequency (Hz) λ: non-sin distortion rate (%) of oscillation waveform

Further, the product of the effective stroke Se and the frequency f represents the length of the slab that is overtaken by the casting mold per unit time. By dividing this product by the casting speed Vc, the area of the slab surface that is overtaken by the casting mold is obtained. The rate is determined (Equation 5 below). The area ratio obtained in this manner is defined as a negative strip area ratio Na (%).

[0049]

## EQU5 ## Na (%) = 100.Se.f / Vc

The effective stroke S obtained as described above
e and the negative strip area ratio Na are merely indexes that mathematically express the interrelationship between the movement of the mold and the slab, so there is no novelty in essence. Attention was paid to the essence of the oscillation function of the negative strip represented by the above, and an attempt was made to formulate an appropriate oscillation condition according to the casting condition using these indices.

The indices representing the casting conditions include the above-mentioned solidification shrinkage index K and the mold sectional shape index L ₁ · Log ₁₀ (L ₂ /
L ₁ ) and the solidus temperature T _SL of the steel were used. As a result of repeating the experiment under various casting conditions, when the following formula 6 is satisfied and the negative strip area ratio Na represented by the above formula 5 is 5 to 100%, the seizure between the mold and the solidified shell is caused. It has been found that it does not occur and that excessive inflow of mold powder can be suppressed.

[0052]

## EQU6 ## Se = e / (T _SL -1418) −g · K + h ·
L ₁ · Log ₁₀ (L ₂ / L ₁ ) where e: coefficient (e = 400 to 1300) g: coefficient (g = 0.2 to 0.3) h: coefficient (h = 5 to 15)

In the above equation (6), e / (T _SL -1)
Item 418) is that when the temperature at which the solidified shell strength is developed (T _SL ) is low and the solidified shell strength is low, the effective stroke S
The term representing the method of preventing seizure between the mold and the solidified shell by increasing the lubrication function of the mold powder by increasing e, and the -g · K term indicates that the solidification shrinkage due to the δ → γ transformation is large. In this case, the gap between the mold and the solidified shell increases, and the inflow of the mold powder becomes excessive. Therefore, a term indicating a method of suppressing the effective stroke Se, and h · L ₁ · Log ₁₀ (L ₂ / The term L ₁ ) increases the effective stroke Se as the solidification shell is more likely to stick to the mold due to bulging as the cross-sectional shape of the mold (solidified shell) becomes flat from a circular shape to a rectangular shape having a longer side, so that seizure occurs more easily. This is a term representing a method of improving lubrication. Note that e, g, and h are adjustment coefficients.

The negative strip area ratio Na is 5%.
If it is less than the value, the negative stripping function of the oscillation becomes insufficient, and seizure between the mold and the solidified shell tends to occur. On the other hand, if it exceeds 100%, a part of the surface of the slab is overlapped and negatively stripped, and the oscillation mark is disturbed and the mold powder is excessively locally introduced to cause uneven solidification. Therefore, in the present invention,. Negative strip area ratio Na of 5
１００100%.

That is, in the second method for controlling the initial solidification of steel according to the present invention, in the continuous casting of ordinary steel, the average mold descending speed U _N (× 10 ⁻³⁾ at the time of the negative strip.
m / sec), the casting speed Vc (× 10 ⁻³ m / sec), and the negative strip time T _N [sec: = 1 / (π · fd) ·
cos ^-1 {Vc / (π · S · fd)}], the effective stroke Se (× 10
^-3 m) is the solidus temperature T determined from the equilibrium diagram of steel.
_SL (K), the above-mentioned solidification shrinkage index K, and the mold cross-sectional shape index L ₁ · Log ₁₀ (L ₂ / L ₁ ) are within the range satisfying the above-mentioned formula 6, and the above-mentioned formula 5
The negative strip area ratio Na (%) represented by
The gist of the present invention is to operate under the oscillation conditions in the range of 5 to 100%.

C. Mold Taper The present inventor tried to express an appropriate value of the mold taper using the above-described solidification shrinkage coefficient K and the mold sectional shape index L ₁ · Log ₁₀ (L ₂ / L ₁ ). As a result, the following conclusions were obtained. This is the third method for controlling the initial solidification of steel according to the present invention.

That is, the initial hardening of the steel according to the third aspect of the present invention.
Solid control method is required in continuous casting of ordinary steel
The outlet size is set to a predetermined value according to the slab cross-sectional shape.
The distance between the inner wall of the mold and the entrance increases from
Meniscus position in a mold with such a taper
In contact with the solidified shell for up to 3 seconds after the start of solidification in
Average taper T_P1(% / M)
The solid shrinkage index K and the above-mentioned mold sectional shape index L₁ ・ Log
_Ten(L_Two / L₁ ), And the value represented by the following equation 7
And the solidification shell 3 seconds after the start of solidification.
Required average taper T at the part in contact with the_P2From 0.6 to 1.3
% / M and the required taper T _P1And required average
Taper T_P2Between the inner wall of the mold when
The difference with the distance between the mold inner walls is 1.5 × 10^-3m
To operate using a mold with a taper
It is assumed that.

[0058]

## _EQU7 ## T _P1 = j · [(2.1 + 0.8 · K) / ｛17
L ₁ · Log ₁₀ (L ₂ / L ₁ ) +1.0｝] where j is a coefficient (j = 0.5 to 1.5)

The reason why the relatively strong taper region shown in Expression 7 is limited to 3 seconds from the start of solidification is that the earliest solidification condition of 3 seconds after the start of solidification most strongly affects the surface quality. . Equation 7 shows a method of giving a large initial mold taper T _P1 in accordance with the solidification shrinkage index K, and a large mold cross-sectional shape index L ₁ · Log ₁₀ (L ₂ / L ₁ ), and the close contact of the solidified shell with the mold. In the prominent case, a large initial mold taper T _P1 is not required, but rather a technique of reducing the taper to prevent seizure between the mold and the solidified shell.

According to various experiments by the present inventors, the required average Taber T _P2 of the portion in contact with the solidified shell after 3 seconds from the start of solidification may be effective when it is 0.6 to 1.3% / m. found. The mold taper after 3 seconds after the start of solidification does not significantly affect the slab surface quality, but 0.6% /
If it is less than m, the adhesion between the mold and the solidified shell may be poor. On the other hand, if it exceeds 1.3% / m, the mold excessively adheres to the solidified shell and the risk of abrasion of the mold increases.

The required taper T described above_P1And required average taper
T_P2In addition to the examination of
Considering the effect of filling the gap with the
Pat _P1Between the inner wall of the mold when
The difference from the distance between the walls is 1.5 × 10^-3m
The difference is filled by filling the powder film, the real harm is
Turned out not to occur.

[0062] Incidentally, the indicated template sectional shape index, the case of a rectangular mold, such as continuous casting of slab, since it is bulging ease of its long side solidified shell, originally described above must taper T _P1 Is to be applied to the long side.

The inventor of the present invention provided various short side tapers to the above-described mold provided with the long side taper according to the present invention, and sought an appropriate short side taper. As a result, when simply considering the ease of bulging, it is thought that a short side with a short side and a high rigidity of the solidified shell should be given a strong taper. It has been found that when an excessively strong taper is applied, the compressive stress applied to the solidified shell is unbalanced, and problems such as buckling deformation of the solidified shell occur.

As a result of trial and error, it was concluded that it was best to give a taper (% / m) of the same degree as the long side. Therefore, it can be said that the taper according to the present invention can be applied to both long and short sides even in the case of a rectangular mold such as continuous casting of a slab.

The mold powder having the above-described composition and physical properties, oscillation conditions, and a mold provided with a taper are:
It is needless to say that when applied individually, effective action and effects are exhibited. However, when the operation is performed by satisfying any two of these at the same time, or when all three are satisfied simultaneously, the operation is more effective. Results can be obtained. This is the fourth method for controlling the initial solidification of steel according to the present invention.

[0066]

EXAMPLES The results of experiments conducted to confirm the effects of the method for controlling the initial solidification of steel according to the present invention will be described below.

Table 1 below shows Examples of the present invention and Comparative Examples. A to H in Table 1 are examples when the method for controlling initial solidification of steel according to the present invention is applied,
In Comparative Examples I to S, any one of the components, the basicity, and the freezing point of the mold powder is out of the claims of the present invention.

In Examples A to H in which all of the components of the mold powder, the basicity, and the solidification point satisfy the claims of the present invention, the slab surface defect index and the slab seizure index (the solidified shell is baked on the mold surface) Index that represents the frequency of
It was less than 0 and did not adversely affect quality or operation. C and F in the embodiment are cast simultaneously under the conditions satisfying claim 2 or claim 3, and embodiments B, D, E, G and H satisfying only claim 1
As compared with, the incidence of both slab surface defects and slab seizure was lower. In particular, in Example A which simultaneously satisfied claims 1 to 3, the improvement effect was remarkable.

On the other hand, the components of the mold powder, the basicity,
In a comparative example in which any of the freezing points is out of the claims of the present invention, the slab surface defect index or the slab seizure index is 1.
Beyond zero, the quality or operation was adversely affected.

Table 2 below shows examples and comparative examples of the present invention according to claim 2. In Table 2, A to J are examples in which the method for controlling the initial solidification of steel according to the present invention is applied, and the comparative examples shown in K to S show that the oscillation conditions satisfy the claims of the present invention. It is out of place.

In Examples A to J in which the oscillation conditions satisfy the claims of the present invention, both the slab surface defect index and the slab seizure index are less than 1.0, which adversely affects the quality and operation. There was no. In Examples, E and I are cast at the same time under the conditions satisfying Claim 1 or Claim 3, and Examples B to D, F to H, and J satisfying Claim 2 only. In comparison, the incidence of both slab surface defects and slab seizure was low. In particular, in Example A which simultaneously satisfied claims 1 to 3, the improvement effect was remarkable.

On the other hand, in Comparative Examples in which the oscillation conditions were outside the scope of the present invention, the slab surface defect index or
The slab seizure index exceeded 1.0, adversely affecting the quality or operation.

Table 3 below shows Examples and Comparative Examples according to the third aspect of the present invention. In Table 3, A to G are examples in which the method for controlling the initial solidification of steel according to the present invention is applied, and the comparative examples shown in H to J indicate that the setting conditions of the mold taper are not claimed in the present invention. It is out of range.

The rectangular molds of Examples C to G and Comparative Examples I and J have tapered long sides and short sides, respectively. Here, Examples C, D, and G and Comparative Examples I and J have a taper. J shows an example of a short side taper, and Examples E and F show examples of a long side taper. As described above, the taper according to the present invention is commonly applied to the long and short sides. 2 to 8 are views showing the taper of Examples A to G and Comparative Examples H to J. The thin solid line indicates the required average taper, and the thick solid line indicates the difference from the required average taper. The taper of the actual mold used for the experiment manufactured as within 5 × 10 ⁻³ m is shown.

In Examples A to G in which the setting condition of the mold taper satisfies the claims of the present invention, both the slab surface defect index and the slab seizure index are less than 1.0, which adversely affects quality and operation. Had no effect. D among the examples is one that was simultaneously cast under the conditions satisfying claim 1, and examples A, B, and E to satisfy only claim 3
As compared with G, the incidence of both slab surface defects and slab seizure was lower. In particular, Embodiment C satisfying Claims 1 to 3 simultaneously
Showed a remarkable improvement effect.

On the other hand, in the comparative example, in setting the mold taper, the necessary taper was set outside the scope of the present invention (H, J) or the actual mold inner wall surface distance (in the casting direction between the opposing mold inner surfaces). This is an example (I) in which the distance (in a direction perpendicular to the direction) deviates by more than 1.5 × 10 ⁻³ m as compared with the case where the necessary taper is provided according to the present invention. In these comparative examples, slab surface defects and slab seizure caused by insufficient mold taper and excessive taper occur frequently.

[0077]

[Table 1]

[0078]

[Table 2]

[0079]

[Table 3]

From the above, selection of a mold powder having appropriate composition and physical properties in accordance with the mold cross-sectional shape, steel type, and casting speed,
It can be seen that quality improvement and operation improvement effects can be obtained by setting the proper oscillation conditions and the proper mold taper setting.

It is needless to say that the effect of improving the initial solidification according to the present invention can be exerted by using each of the mold powder, the oscillation conditions, and the mold taper alone. However, when these are carried out in combination, a further synergistic effect can be obtained. , It is desirable to implement at least two or more items simultaneously.

[0082]

As described above, the method for controlling the initial solidification of steel according to the present invention can be universally applied to a wide range of mold shapes, mold sizes, and steel types, and can be used to reduce the slab surface defects and slabs. Seizure is effectively suppressed, and quality improvement and stable operation are possible.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory view of a negative strip in continuous casting of steel, in which (a) and (c) represent amplitude on the vertical axis and time on the horizontal axis, and (b) differentiates the vertical axis of (a). (A) shows a case of a sin waveform, and (c) shows a case of a non-sin waveform.

FIG. 2 is a diagram showing a required average taper and an actual taper of Example A and Comparative Example H in Table 3.

FIG. 3 is a diagram showing a required average taper and an actual taper of Example B in Table 3.

FIG. 4 is a diagram showing a required average taper and an actual taper of Example C in Table 3.

FIG. 5 is a diagram showing a required average taper and an actual taper of Example D in Table 3.

FIG. 6 is a diagram showing a required average taper and an actual taper of Example E in Table 3.

FIG. 7 is a diagram showing a required average taper and an actual taper of Example F in Table 3.

FIG. 8 is a diagram showing a required average taper and an actual taper of Example G and Comparative Example J in Table 3.

FIG. 9 is a diagram showing a required average taper and an actual taper of Comparative Example I in Table 3.

Claims (4)

[Claims] 1. A continuous casting of ordinary steel, comprising F as a main component of CaO.SiO ₂ ,
One of the main crystal compositions is caspidin (3CaO.
2SiO ₂ · CaF ₂ ), the basicity of the mold powder expressed by the mass concentration ratio of CaO, MgO, SiO ₂ , Al ₂ O ₃ , and TiO ₂ ｛(CaO + 1.2MgO) /
(SiO ₂ +0.5 Al ₂ O ₃ +0.7 TiO ₂ )}
The shortest distance from the center of the mold section to the inner wall of the mold is L
₁ (m), and assuming that the longest distance from the center point of the mold section to the inner wall of the mold is L ₂ (m), L ₁ · Log ₁₀ (L ₂ / L ₁
To the template sectional shape index represented by), represented by the following formula, The composition, expressed in weight concentration of the other ingredients, I _A metals oxide ≦ 10%, F ≧ 1.5% , ZrO 2 ≦ 5%, MnO
≦ 5%, B ₂ O ₃ ≦ 5%, the solidification point Sp (K) is the solidus temperature T _SL (K) obtained from the equilibrium diagram of the steel, and the transformation from δ phase to γ phase The solidification shrinkage index K expressed by the following equation using the temperature T _δγ (K) at the intermediate point between the start point and the end point of the _solid phase, the volume ratio α (%) of the δ phase at the time of complete solidification, and the casting speed Vc ( × 10 ^{- 3} m / sec.) using the following formula, the initial controlled solidification method of steel, characterized by using a mold powder having the composition and physical properties. (CaO + 1.2MgO) / (SiO ₂ + 0.5Al ₂ O ₃ + 0.7TiO ₂ ) = a · [1.0 · {L ₁ · Log ₁₀ (L ₂ / L ₁ )} ^0.3 +1.0] a: Coefficient (a = 0.6 to 1.1) Sp = T _SL −b + c ^K + d · Log ₁₀ (0.06 · Vc) K = α / {T _SL − (T _δγ −5)} where , B: coefficient (b = 300 to 400) c: coefficient (c = 3.0 to 3.5) d: coefficient (d = 40 to 50) 2. In a continuous casting of ordinary steel, an average mold descending speed _UN (× 10 ⁻³ m / sec) and a casting speed V during a negative strip represented by the following formula:
c (× 10 ⁻³ m / sec) and the negative strip time T
_N [seconds: = 1 / (π · fd) · cos ⁻¹ ｛Vc / (π ·
S · fd)｝], the effective stroke Se (× 10 ⁻³ m) represented by the following equation is calculated from the solidus temperature T _SL (K) determined from the equilibrium diagram of steel. And the solidification shrinkage index K represented by the following formula and the mold sectional shape index L ₁ · Log ₁₀ (L ₂ / L ₁ ) according to claim 1 are within a range satisfying the following formula: A method for controlling initial solidification of steel, comprising operating under an oscillation condition in which a negative strip area ratio Na (%) represented by the following formula is in the range of 5 to 100%. _{Se = (U N -Vc) ·} T N ... U N = S / T N · sin ^{[cos -1 {-Vc / (π ·} S · fd)} ] ... However, fd = f / (1- λ / 100) S: Oscillation full stroke (× 10 ⁻³ m) f: Oscillation frequency (Hz) λ: Non-sine distortion rate (%) of oscillation waveform Se = e / (T _SL -1418) −g · K + h · L ₁ · Log ₁₀ (L ₂ / L ₁ ) where e: coefficient (e = 400 to 1300) g: coefficient (g = 0.2 to 0.3) h: coefficient (h = 5 to 15) ) Na (%) = 100 · Se · f / Vc ... 3. In a continuous casting of ordinary steel, in a mold provided with a taper such that a predetermined outlet dimension is set according to a required slab cross-sectional shape, and a distance from an inner wall facing the outlet increases from an outlet to an inlet. The required average taper T _P1 (% / m) of a portion in contact with the solidified shell during 3 seconds after the start of solidification at the meniscus position is determined.
The solidification shrinkage index K represented by the following formula and the mold sectional shape index L ₁ · Log ₁₀ (L ₂ / L ₁ ) according to claim ₁ .
And the required average taper T _P2 of the portion in contact with the solidified shell after 3 seconds from the start of solidification is determined to be 0.6 to 1.3% / m, and The taper is provided so that the difference between the distance between the inner walls of the mold when the necessary taper T _P1 and the required average taper T _P2 are provided and the actual distance between the inner walls of the mold is within 1.5 × 10 ⁻³ m. A method for controlling the initial solidification of steel, wherein the method is operated using a set mold. T _P1 = j · [(2.1 + 0.8 · K) / {17 · L ₁ · Log ₁₀ (L ₂ / L ₁ ) +1.0}] where j: coefficient (j = 0.5 to 1) .5) 4. A mold powder having the composition and physical properties according to claim 1, at least two of the oscillation conditions according to claim 2 and the mold provided with a taper according to claim 3 at the same time. A method for controlling the initial solidification of steel, characterized by satisfying and operating.