JPH11291005A - Mold flux for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid the generation of surface defect on a cold-rolled plate caused by the entrapment of mold powder and to prevent the generation of breakout and misalarm of the breakout caused by the shortage of lubrication, in a continuous casting of a steel. SOLUTION: The mold flux for continuously coasting the steel is the one, in which the solidified temp. is 1100-1250 deg.C and a basicity index B defined with the equation is satisfied in the range of 1.7-2.2 and Al2 O3 is contained in the range of 4-10 wt.%. Then, the casting speed in the continuous casting is made to >=1.5 m/min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold flux for continuous casting of steel, and more particularly, to a powder flux even when continuously casting ultra-low carbon steel typified by a steel sheet for automobiles, and even when continuously casting at high speed. It is intended to advantageously avoid deterioration of the surface quality of the product plate due to defects.

[0002]

2. Description of the Related Art Mold powder for continuous casting of steel is made of Si.
It is composed mainly of O ₂ , CaO, Al ₂ O ₃ , Na ₂ O, NaF, CaF ₂ and MgO, etc., depending on the casting speed of steel, its viscosity,
The softening temperature and solidification temperature are adjusted. This mold powder is an important auxiliary material in the continuous casting process and is added on the surface of molten steel injected into the mold,
It mainly plays the role of lubrication between the mold and the solidified shell, absorbing and removing inclusions in the steel that floated up, keeping the temperature of the molten steel, and preventing oxidation. However, on the other hand, the molten powder on the molten steel is entrained in the molten steel injection flow in the mold, or adheres to the solidified shell due to local fluctuations in the molten metal surface, and is extended to the rolling process as it is, and the surface of the cold-rolled steel sheet is extended. There was a case where it was defective.

As a method for preventing powdery defects in a cold-rolled steel sheet, for example, Japanese Patent Application Laid-Open No. 4-224063 discloses a technique for preventing entrapment of powder by setting the viscosity of mold powder to about 2.0 Poise or more. ing. However, simply increasing the viscosity reduces the entrapment of the molten powder during melting, but does not eliminate any defects in the cold-rolled steel sheet, and high-viscosity powder lowers the upper limit of casting speed. This is an obstacle to productivity.

Japanese Patent Application Laid-Open No. 4-200962 discloses that an exothermic agent is added to a mold powder to prevent the temperature of the upper surface of molten steel from dropping, so that inclusions and air bubbles adhere to an abnormally developed solidified shell. There is disclosed a technique for preventing such a situation. However, in this method, since a raw material that generates an alkali metal gas is used as a heat generating agent, smoke is remarkable, and adverse effects on the sanitary environment of workers cannot be overlooked.

Other prior art techniques for preventing abnormal development of the solidified shell include increasing the basicity of the mold flux and the solidification temperature, increasing the solid phase thickness in the mold flux layer between the mold and the slab, and increasing the radiant heat flux. (CAMP-ISIJ,
5 (1992), p. 283) and maintaining the basicity of the mold flux at about 1.2, adding about 3% of ZrO ₂ , and further increasing the freezing point to reduce radiant heat transfer and increase the mold temperature. There is known a technique for reducing the cooling by increasing the contact thermal resistance between the layer and the solidified layer of the flux to prevent cracks (see CAMP-ISIJ, 6 (1993), p. 283).

However, the above-mentioned CAMP-ISIJ, 5 (19)
92), P.283 and CAMP-ISIJ, 6 (1993), the conventional technique using a high freezing point flux as disclosed in P.283, the solidification temperature is as high as about 1200 ° C, so that the space between the mold and the slab is Since the thickness of the liquid phase portion of the flux film to be interposed in the mold cannot be sufficiently ensured, lubrication failure occurs and breakout is likely to occur, and mold flux inflow from the middle to lower part of the mold tends to be poor. Partial changes in temperature are likely to occur. Therefore, when a breakout alarm system of a type that measures the temperature of the mold is introduced, a false alarm is generated at a high frequency, which may disturb the operation and reduce the productivity. Furthermore, due to the high solidification temperature, a fixed layer of flux called a slag rim develops thickly and non-uniformly right above the so-called meniscus where the upper surface of the molten steel and the mold come into contact with each other. In some cases, it interferes with the inflow of the solidified shell and causes non-uniform inflow, which may cause restraint-breakout of the solidified shell.

[0007] In addition, the above-mentioned tendency of surface quality deterioration and breakout due to powder defects is remarkable when the target steel type is extremely low carbon steel or the casting speed is high.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to prevent the entrainment of mold powder in continuous casting of steel as much as possible while effectively suppressing the abnormal growth of the solidified shell. Prevent adhesion to the solidified shell, thereby advantageously avoiding the occurrence of surface defects in cold-rolled steel sheets, and at the same time eliminate breakouts and breakout false alarms, and efficiently produce continuous cast slabs There.

[0009]

Means for Solving the Problems Now, the present inventors have conducted various experiments and studies on the relationship between the suppression of heat transfer in the mold and the solidified structure, and the lubricity of the mold and the slab.
By properly adjusting the component composition of the flux, not only can the heat transfer between the solid phase portion and the mold wall surface of the mold powder layer interposed between the solidified shell and the mold be effectively suppressed, while obtaining sufficient lubrication. With new knowledge that mold flux entrainment can be advantageously prevented,
The present invention has been completed.

The findings are the following five. (1) When the liquid phase mold flux comes into contact with the mold surface and instantaneously solidifies into a glassy state and the glass phase is stable (lower part in Fig. 1), heat transfer at the interface between the mold and the glassy flux The resistance is very close to zero. At this time, the roughness Rmax of the contact surface of the glass mold flux with the mold is several μm.
m or less. On the other hand, when the liquid mold flux comes into contact with the mold surface and crystal nuclei are generated directly on the contact surface and grow and solidify, the interface heat transfer resistance is about 0.1 to 0.3 (m ² / kW).
At this time, the roughness Rmax of the contact surface with the mold is about 10 μm or less.

(2) When the liquid phase mold flux contacts the mold surface and instantaneously solidifies into a glassy state, and then sufficiently precipitates in the glass phase and the precipitated phase reaches the mold surface (upper part in FIG. 1). , The interface heat transfer resistance is 0.2 to 0.5 (m ² / kW)
Degree, and the roughness Rmax of the contact surface with the mold at this time is 15 μm.
m or less.

(3) The roughness of the contact surface of the mold flux film with the mold and the apparent thermal conductivity of the mold flux film substantially correspond to one another, and the higher the roughness, the lower the thermal conductivity. . And the lower the minimum crystal precipitation temperature in the glass phase, the more stable the mold flux solidified layer in contact with the mold surface in the crystal phase, and the more uniform the interface heat transfer resistance with the mold (0.3 m ² / kW). .

(4) The basicity index B represented by the following formula is 1.7
As described above, when the mold flux having a solidification temperature of 1100 ° C. or more is used, the state (3) can be stably created. Record

(Equation 2)

(5) When an appropriate amount of Al ₂ O ₃ is added to the mold flux, the temperature dependence of the viscosity is reduced, and a decrease in the viscosity at a high temperature can be suppressed.

The present invention is based on the above findings. That is, according to the present invention, the solidification temperature is 1100 ° C or higher,
At 1250 ° C or less, the basicity index B defined by the following formula is 1.7
Not less than 2.2 and not less than 4 wt% of Al ₂ O ₃ and 10 w
It is a mold flux for continuous casting of steel, which is contained in a range of t% or less. Record

(Equation 3)

In the present invention, the viscosity of the mold flux preferably satisfies the range of 2 Poise or less at 1300 ° C. and 0.5 Poise or more at 1500 ° C.

Further, the mold flux of the present invention comprises:
Used for high-speed continuous casting with a casting speed of 1.5 m / min or more in continuous casting, and used under conditions where there was a concern that powder defects would occur in product sheets, such as ultra-low carbon steel. Even in such a case, a desired effect can be obtained, which is particularly advantageous.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. First, a situation in which a solidified shell is being formed in a so-called meniscus upper region in the continuous casting mold will be described. The heat removal of the molten steel and the solidified shell in the mold is all performed via the mold flux between the solidified shell and the mold. A solid phase layer (solidified layer) is formed at the portion of the mold flux that is in contact with the mold, and a liquid phase is formed at the portion that is in contact with the solidified shell because the temperature exceeds the solidification temperature of the flux. The heat transfer through this flux layer, in terms of its contribution,
50% or more is determined by the heat transfer resistance between the mold and the flux solidification layer, that is, the state of contact between the two.

Therefore, for example, by using a high freezing point flux as in the prior art, a crystal layer is formed from the liquid phase on the mold side, and a thermal resistance with the mold surface corresponding to the unevenness of the solidified layer of about 10 μm is formed. , Uniform slow cooling is achieved. However, in this method, since the appropriate liquid phase thickness cannot be secured, lubrication becomes unstable, and furthermore, due to the high solidification temperature of the flux, a flux called a slag rim is placed immediately above the so-called meniscus where the molten steel upper surface and the mold come into contact. The thick fixed layer develops thickly and non-uniformly, which interferes with the molten flux between the mold and the slab during the vibration of the mold to cause uneven inflow, which may cause a breakout.

For this reason, a low-viscosity flux must be designed. On the other hand, such a low-viscosity flux is easily caught in the molten steel due to the flow of molten steel in the mold, and non-metallic inclusions in the cold-rolled sheet. It is one of the causes of the caused defects.

The inventors have conducted intensive studies to solve the above problems. As a result, it is not enough to simply define the freezing point to obtain a good flux.

(Equation 4) It has been found that it is extremely important to limit the basicity index B defined in the above to the range of 1.7 to 2.2. here,
The denominator in the above formula is a component that promotes crystallization (network former), while the numerator is a component that inhibits crystallization (network breaker).

By adjusting the value of the basicity index B to the above range, when the liquid phase mold flux comes into contact with the inner wall of the mold and vitrifies, the precipitation of crystals proceeds smoothly in this glass phase, The roughness Rmax of the contact surface with
As a result, a relatively uniform interfacial heat transfer resistance of about 0.3 m ² / kW is obtained between the mold and the mold, so that a stable and uniform slow cooling is realized and, consequently, abnormal development of the solidified shell. Is suppressed. Further, when the value of B satisfies the above range, the temperature range where lubricity is required (representative temperature: 13
(00 ° C), the viscosity is relatively low, while the viscosity can be relatively high in the temperature range (representative temperature: 1500 ° C) where there is concern about entrainment. Good lubricity can be secured between the solidified shells.

However, if the value of the basicity index B is less than 1.7, the slow cooling effect is not sufficient and the surface defect index increases, whereas if it exceeds 2.2, the quality is maintained good, but the crystallization is reduced by lubrication in the mold. Therefore, it is important to limit the value of the basicity index B to a range of 1.7 to 2.2, since the basicity index B starts to influence the breakout prediction index. Regarding the viscosity characteristics of the flux, the viscosity at 1300 ° C is 2 Poise or less, and the viscosity at 1500 ° C is 0.5 Poise.
It is preferable to make the above.

The basicity index B and the viscosity of the flux are calculated as follows:
By adjusting to the above range, the mold flux solidified layer in contact with the mold is rapidly and uniformly crystallized, the heat transfer resistance between the mold and the flux film becomes uniform, and it is possible to cool slowly and effectively. As a result, abnormal development of the solidified shell can be suppressed, and trapping of the entrained mold flux in the slab can be prevented.

However, in this case, the desired effect may not always be obtained because the viscosity has a large temperature dependency. In this regard, the appropriate amount of A
This can be solved by adding l ₂ O ₃ . However, the addition of Al ₂ O ₃ is 4 wt% to not Still large temperature dependence of viscosity less than, can not be a value sufficient to suppress entrainment of the viscosity at 1500 ° C., whereas Al ₂ O ₃ added If the amount exceeds 10 wt%, Al ₂
Since heat flux and lubricating properties of the flux are remarkably changed due to concentration in the mold flux due to floating of O ₃ , and stable casting is hindered, Al ₂ O ₃ is added in the range of ₄ to 10 wt%.

The mold flux of the present invention is:
In addition to the components listed in the formula for defining the basicity index B, MnO, Fe ₂ O ₃ , TiO _2, ZrO _{2, and the} like which are usually used as a flux may be included.

[0027]

EXAMPLES Continuous casting was carried out in the following manner using mold flux having the composition shown in Table 1.・ Mold size Long side: 1000mm, Short side: 200mm ・ Mold vibration condition: Stroke: 8mm Negative speed ratio (Ns ratio): 20% (Ratio of lowering speed of mold due to vibration of mold exceeding pulling speed of slab) f = Vc / 2 × 8 (1 + Ns / 100) × 1000 where f: mold frequency (cpm) Vc: casting speed (m / min) casting speed: 1.20 to 2.20 m / min Difference from liquidus temperature ΔT: 34 ~
50 ° C (typical) ・ Steel cast steel (C: 0.002 to 0.004 wt%, Si: 0.02 to 0.
04wt%, Mn: Ultra low carbon steel containing 0.10 ~ 0.30wt%)

Table 2 shows the predictive index of restrictive breakout when continuous casting was performed under the above conditions. Here, the breakout prediction index is the cumulative casting when the number of times that the shell fracture is confirmed in the examination of the slab side is N for the solidification shell fracture or breakout warning predicted by the thermocouple embedded in the molded copper plate. For N corresponding to length L, N
/ L in exponential notation. Table 2 also shows defects caused by the flux of the cold rolled sheet obtained by performing hot rolling and cold rolling after continuous casting. Table 2 also shows various physical property values of each flux.

[0029]

[Table 1]

[0030]

[Table 2]

As shown in Table 2, when the mold flux according to the present invention was used, there was no surface defect caused by the flux, and even if it occurred, it was very slight. Furthermore, the number of predictions of the restraint breakout was also indicated by an index, and was able to be reduced to about 1/10 or less of the mold flux having the same degree of slow cooling.

[0032]

As described above, according to the present invention, good lubricity can be ensured while effectively preventing the mold flux from being entrained, so that the surface quality of the product plate deteriorates due to the powder defect. And breakouts can be effectively prevented.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a heat transfer mechanism between a mold and a mold flux.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasuo Kishimoto 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Research Institute Co., Ltd. (72) Inventor Sawao Ishikawa 3-chome Oikecho, Suma-ku, Kobe-shi, Hyogo No. 1-26 Sakai Chemical Industry Co., Ltd. (72) Inventor Takahiro Mitsumune 3-26 Oikecho, Suma-ku, Kobe City, Hyogo Prefecture Sakai Chemical Industry Co., Ltd.

Claims (4)

[Claims] Claims: 1. A solidification temperature of 1100 ° C. or more and 1250 ° C. or less
A mold flux for continuous casting of steel, wherein the basicity index B defined by the following formula satisfies 1.7 or more and 2.2 or less and contains Al ₂ O ₃ in a range of 4 to 10 wt%. . Note 2. The method according to claim 1, wherein the viscosity at 1300 ° C. is 2 Poise or less and the viscosity at 1500 ° C. is 0.5 Poi.
Mold flux for continuous casting of steel with se or higher. 3. The mold flux for continuous casting of steel according to claim 1, wherein the casting speed in continuous casting is 1.5 m / min or more. 4. The mold flux for continuous casting of steel according to claim 1, 2 or 3, wherein the target steel type in continuous casting is an ultra-low carbon steel.