JPH0712531B2 - Method for preventing restraint breakout during continuous casting - Google Patents

Description

The present invention relates to a restraint breakout preventing method during continuous casting, and in particular, by limiting the viscosity of the mold powder to be charged on the surface of molten steel in a mold, Ar gas A method for preventing restraint breakout during blowing is widely used in the field of continuous casting of steel.

[Conventional technology]

In recent years, with the progress of continuous casting technology for steel, a method for changing the continuous width of a slab slab during casting, a so-called continuous casting method for different steel types, and the like have been developed, and remarkable improvement in productivity has been achieved. However, the application of these technologies is limited due to the variety of mold steel types and the restrictions on the amount of casting, so recently development efforts have been focused on improving continuous casting capacity by reducing casting accidents such as breakouts. ing.

During an accident during continuous casting, especially if a breakout occurs once, it is forced to stop the casting work, and it takes several hours to restore it, which significantly impairs productivity. Breakout is a lubrication failure of the mold powder that acts as a lubricant by flowing into the void formed between the mold and the mold due to the contraction of the solidified shell formed by the solidification of molten steel that contacts the mold during continuous casting It is a phenomenon that the solidified shell once generated breaks and adheres to the inner wall of the mold, and the unsolidified molten steel inside leaks out by being released from the normal solidified shell pulled out by the pinch roll. In general, this type of breakout is called a binding breakout.

Mold powders are generally CaO, SiO ₂ , Al ₂ O ₃ , Na ₂ O,
It has a chemical composition of K ₂ O, C, F, Fe ₂ O _3, etc., and is a granular or powdery substance containing 30 to 40% of CaO and SiO ₂ , respectively. When charged, it rapidly becomes a molten steel state, prevents oxidation of the molten steel, fills the space between the formed solidified shell and the inner wall of the mold, and exists in the form of a film, and acts as a lubricant when the cast piece descends. However, when the fluidity of the mold powder is insufficient, the friction between the solidified shell and the wall surface of the mold causes breakage as described above, resulting in breakout.

Since there is no direct detection means for breakout, a conventional means for preventing this is, for example, JP-A-57-52.
A method of preheating mold powder as disclosed in 556, thereby rapidly melting the powder dispersed in the mold to obtain a slag film having a sufficient thickness, or JP-A-52- As disclosed in 26318,
A method and the like have been proposed in which the viscosity of mold powder is adjusted according to the casting speed to perform continuous casting.

However, when pursuing the cause of the constrained breakout, there are many phenomena that cannot be explained by the means adopted in the casting method. For example, in a low-speed casting with a casting speed of 1 m / min or less, the mold powder put into the mold is quickly melted, and a breakout may occur even under the condition that an appropriate slag film is formed. It may occur even though the casting speed and powder used do not change.

The phenomenon that the present inventors have focused on as a cause of frequent occurrence of restraint breakout is as follows.

(A) In the ladle process after tapping the converter, restraint breakouts tend to occur more frequently when the flushing process is performed with Ar gas or the like than when the vacuum degassing is performed.

(B) To prevent nozzle clogging due to Al ₂ O ₃ etc. when using the immersion nozzle from the tundish to the mold A
r It tends to occur frequently when blowing gas.

Regarding the phenomenon of (a) above, hydrogen in the steel is considered to be the cause, and the phenomenon of (b) is due to gas foaming based on blown Ar, and the effects of gas foaming on the mold powder have been studied repeatedly. As a result, the present invention is related.

[Problems to be solved by the invention]

The purpose of the present invention is to regulate the mold powder to be added to the surface of the molten steel in the mold during continuous casting, even when the molten steel used is subjected to the flushing treatment with Ar gas, and also to prevent immersion nozzle clogging from the Danishitsuyu. Therefore, it is an object of the present invention to provide a restraint breakout prevention method during continuous casting that does not generate restraint breakout even when Ar gas is blown, and always enables stable continuous casting.

[Means and Actions for Solving Problems]

The gist of the present invention is as follows. That is, in the restraint breakout prevention method during continuous casting that adjusts the viscosity of the mold powder that is put into the molten steel surface in the mold during the continuous casting of steel, in the viscosity comparison at a temperature 50 ° C lower than the solidification temperature of the mold powder when the viscosity ...... P ₂ when there is no blow or the viscosity ......... P ₁ argon gas in the case that crowded blowing the argon gas in the powder g per 1cc in the powder, A method for preventing restraint breakout during continuous casting characterized by using a mold powder satisfying the following relational expression.

As a result of various studies conducted by the present inventors on the cause of the constrained breakout, they found that the crystallization of the mold powder had the greatest effect of significantly deteriorating the lubricity. That is, the mold powder during casting is an oxide-based melt, which flows into the gap between the vibrating mold and the solidified shell, and smoothes the slab having the solidified shell formed in the form of glass in the cooling process. The normal action is to become a lubricant that can be lowered to. However, when crystals are precipitated during the cooling process of the mold powder, it is considered that this lubricating function is remarkably deteriorated, and as a result, the solidified shell is broken by friction and breakout occurs.

As a result of investigating the precipitation of crystals in the cooling process of the mold powder, when H in the steel was 7 ppm or more, or Ar was blown into the molten steel to prevent Al ₂ O ₃ from adhering to the immersion nozzle from the tan dish. It was found that the crystal deposition was promoted due to the Ar gas being trapped by the mold powder when it was incorporated.

With respect to the above, the experimental results conducted by the present inventors regarding the amount of Ar gas blown into the molten steel and the relative degree of crystal precipitation will be described. That is, the present inventors, 150g sample of mold powder in a graphite crucible was heated and melted at 1300 ° C for 30 minutes, poured into a cylinder, and Ar gas was passed through the silica tube into the molten powder for 10 seconds. When it was left to stand and solidified, and after cooling, the slag was crushed and examined by X-ray diffraction. As a result, a peak of caspidine (3CaO.2SiO ₂ .CaF ₂ ) was observed. When the test was performed by changing the blowing amount of Ar gas in this way, the relationship between the blowing amount of Ar gas (cc / g) and the relative degree of crystal precipitation observed in the solidified powder was as shown in Fig. 2. Was found to exist. That is, as the Ar amount increases, the number of Ar gas bubbles 6 captured increases, and the degree of precipitation of the crystals 8 increases.

In addition, the temperature measurement experiment of the mold powder at the time of blowing Ar gas was performed.
Transfer the molten powder at 00 ℃ to a crucible, change the Ar gas flow rate, blow Ar gas for 10 seconds, and measure the temperature of the molten powder in each case, and also when Ar gas is not blown at all. Similarly, the temperature was measured and shown in FIG. 3 at the same time.

As is clear from FIG. 3, the sample A that does not blow Ar gas is 75
While showing a temperature of 0 ° C., sample B in which Ar gas was blown,
It can be seen that all of C, D, and E are 850 ° C. or higher, and the temperature drop of the molten powder at about 100 ° C. is hindered and the powder is gradually cooled.

This is because when Ar gas is blown, Ar gas is trapped in the molten powder as bubbles, and the trapped Ar gas bubbles prevent thermal diffusion, resulting in slow cooling and accelerated crystallization. Conceivable.

As described above, it is considered that when the crystallization of the molten mold powder is promoted, the fluidity is reduced, the action as a lubricant is significantly impaired, and a frictional breakout occurs due to friction.

This fact is supported by the following phenomena observed in continuous casting operations.

(A) Immediately before breakout, disturbance of the occlusion mark is observed on the slab. This indicates poor lubrication of the powder.

(B) The powder film just before the breakout was cloudy. This is considered to be due to the concentration of large and small bubbles.

(C) Just before the breakout, the consumption of mold powder is low. This is considered to be due to the decrease in liquidity.

From the above experimental results of the present inventors, in continuous casting, the fluidity is lowered by promoting crystallization in the molten powder present in the film-like form in the gap between the mold and the solidified shell, and the cause of breakout due to friction However, the detailed mechanism leading to the breakout is still unknown, but it can be considered as follows. That is, when the solid-liquid interface morphology in the mold powder layer is crystal solidification, it becomes complicated as a dendrite, and dispersion of solid-phase nuclei occurs in the liquid phase, resulting in an increase in viscosity in the liquid phase. To be As described above, the bubbles of Ar gas are trapped in the liquid phase of the mold powder, which hinders thermal diffusion and is gradually cooled. As a result, crystals are precipitated and the powder viscosity increases, resulting in poor lubrication and friction. It is thought that this will lead to a restrictive breakout.

As a result of various investigations conducted by the present inventors on the method for quantitatively evaluating the crystallization tendency of such mold powder by Ar gas, the following method was found to be suitable. That is,
A method of measuring the viscosity of powder while introducing 10 cc / min of Ar gas per 1 g of mold powder during solidification, comparing this with the viscosity when Ar gas is not blown at all, and evaluating by the rate of change Is. However, as will be described later, it has been found that it is most suitable that the measurement temperature is lower than the solidification temperature of the mold powder by 50 ° C.

Therefore, it has been found that the viscosity value can be substituted by the torque value which is correlated with the viscosity. The inventors of the present invention measured the torque value of the mold powder with a measuring device as shown in FIG. That is, an alumina crucible 16 having an inner diameter of 40 mm, an outer diameter of 44 mm, and a depth of 50 mm is placed in a heating furnace 14 surrounded by a refractory brick 12 having a heating element 10, while an opening in the upper central portion of the heating furnace 14 is placed. Insert a stainless stirrer rod 22 that is rotated by a motor 20 through 18 and rotate a repeller 24 attached to the tip of the stainless steel rod in the crucible 16 while measuring the temperature with a thermocouple 26.
The torque value of the mold powder 2 heated in the crucible 16 was measured by a torque meter 28 provided at the upper part of the outside of the furnace 22 when Ar gas was blown into the mold powder 2 and when the blowing was not performed.

Using the above torque value measuring device, for the test mold powder with CaO / SiO ₂ = 0.96 and solidification temperature 1130 ° C, the torque value T ₁ when Ar gas is blown and the torque when Ar gas is not blown at all The value T ₂ was measured. When using the test powder, in the case of blowing Ar gas as compared with the case of not blowing Ar gas, the heating value is gradually lowered from the solidification temperature of 1130 ° C., the torque value gradually rises, At a temperature 50 ° C lower than the solidification temperature, the rate of change of the torque value with respect to the torque value T ₂ when Ar gas is not blown is The propellers are starting to rotate from this temperature. As described above, the rate of change of the torque value when Ar gas is blown and when it is not blown generally appears clearly in the low temperature range of 30 to 70 ° C. from the solidification temperature of the test powder, and therefore the present invention solidifies the mold powder. It was decided to use the torque value at a temperature 50 ° C lower than the temperature as a reference. Thus, regarding the test powder, the torque value was continuously measured in the temperature range from the solidification temperature of 1130 ° C to the low temperature of 100 ° C up to 1030 ° C when Ar gas was not blown and when 1 cc of Ar gas was blown per g of powder. The measurement results are as shown in FIG.

Since the above torque value has a correlation with the viscosity of the mold powder in the molten state, the rate of change of the above torque value with respect to the case where Ar gas is not blown Is the rate of change of viscosity Can be replaced with

Next, the present inventors have prepared four types of test mold powders A and B having the composition shown in Table 1 and the characteristics shown in Table 2.
Using C and D, the viscosities P ₁ and P ₂ were measured for each case in which 1 cc of Ar gas was blown in per g of powder and no Ar gas was blown at 50 ° C. lower than the solidification temperature. , Viscosity when Ar gas is not injected at all P ₂
Change rate of viscosity The above four brands of A, B, C, and D having different viscosity change rates due to the above were used at the time of continuous casting in actual operation, and the occurrence rate of restraint breakout was investigated. The results are shown in Fig. 6.

From Fig. 6, powders A and B have a viscosity change rate of 70-100%.
It was found that while the breakout occurrence rate was extremely high, powders C and D had a viscosity change rate of 25% or less, and the breakout occurrence rate was extremely low.

In the test mold powders of brands A, B, C, and D shown in Tables 1 and 2, brands A and C2 have manufacturing thickness of 22.
Powder used in a continuous casting machine of 0 mm x width 850 to 1500 mm, which is operated at a casting speed of 1.0 to 1.8 m / min, while the B and D2 brands have manufacturing dimensions of thickness 215 to 310 mm x width 14
It is a powder used in a continuous casting machine of 50 to 2500 mm and is operated at a casting speed of 0.4 to 0.9 m / min.
As described above, since the manufactured slab size is different and the casting speed is different, the A and C2 brands with low viscosity at 1300 ° C. are used, and the B and D2 brands with high viscosity are selected and used.

As described above, in the present invention, the viscosity change rate with respect to the viscosity P ₂ when Ar gas is not blown at a temperature 50 ° C. lower than the solidification temperature of the mold powder used during continuous casting And decided to limit.

In the actual operation, the inventors were able to reduce the restraint breakout to almost zero by selecting the mold powder or adjusting the components according to the above limitation.

As a method for suppressing the increase in viscosity of the mold powder due to blowing of Ar gas, the following characteristics of the mold powder may be aimed.

(A) Basicity CaO / SiO ₂ is lowered to preferably 0.90 or less. (B) BaO and MgO are added.

(C) Select a brand with a low softening temperature and solidification temperature.

Among the above characteristics, (a) and (b) are particularly effective.

〔The invention's effect〕

The present invention clarifies that the probability of occurrence of constrained breakout in continuous casting is extremely high depending on the characteristics of the mold powder scattered in the mold, and researches on the setting of a criterion for limiting the suitability of mold powder for preventing breakout. As a result, the freezing temperature of the mold powder is 50 ℃
When comparing the viscosity at low temperature, 1 cc per g of the powder
It was found that this can be achieved by adjusting the composition so that the viscosity change rate between the case where Ar gas is blown in and the case where Ar gas is not blown is 25% or less,
I was able to list the following effects.

(A) The application of the present invention can significantly reduce the occurrence of restraint breakout, and can contribute to a remarkable improvement in productivity and a reduction in cost.

(B) Since the present invention only requires adjustment of the components of the mold powder, it is sufficient to previously mix the mold powder of the optimum component and use this, the method is simple, and the effect is extremely great.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the cross section of the mold powder in which Ar gas is blown in a molten state after cooling and solidification, and FIG. 2 is the mold powder in the experiment for obtaining the present invention, which is blown during heating and melting at 1300 ° C. Correlation diagram showing the effect of Ar gas amount (cc / g powder) on the relative degree of crystal precipitation after solidification.
Correlation diagram showing the relationship between the amount of Ar gas blown into the mold powder being heated and melted at 1300 ° C. and the temperature change of the mold powder during cooling, and FIG. 4 is the torque value measurement of the mold powder used by the inventors. Fig. 5 is a cross-sectional view showing the equipment. Fig. 5 shows the test mold powder from the solidification temperature of 1130 ℃ to 100 ℃.
In the low temperature range up to 1030 ℃, 1 cc of Ar gas is injected per g of powder (marked with ●) and not blown (○)
Fig. 6 shows the change in the torque value at the mark), and Fig. 6 shows the change in viscosity when 1 cc of Ar gas is blown per g of powder and when no Ar gas is blown at 50 ° C lower than the solidification temperature of the test mold powder. It is a correlation diagram which shows the influence which the difference of a rate has on a restraint breakout occurrence rate. 2 ... Mold powder, 4 ... Glass part 6 ... Bubbles, 8 ... Crystal

Claims (1)

[Claims] 1. A method for preventing a constrained breakout during continuous casting, which comprises adjusting the viscosity of a mold powder that is put on the surface of molten steel in a mold during continuous casting of steel, wherein the viscosity at a temperature 50 ° C. lower than the solidification temperature of the mold powder. when the viscosity ...... P ₂ in the absence blow or the viscosity ......... P ₁ argon gas when forme blowing argon gas in the powder g per 1cc in the mold powder in the comparison, A method for preventing restraint breakout during continuous casting, characterized by using a mold powder satisfying the following relational expression.