JP3137826B2 - Continuous casting method and mold lubricant for continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method and a mold lubricant for continuous casting. That is, in the present invention, in the continuous casting of steel, powder or granules are added to the molten steel surface in the mold to prevent solidification of the molten steel surface while maintaining the temperature of the molten steel surface in the mold, and non-metallic intermediates that have floated on the molten steel surface. The slag film flowing between the mold and the solidified shell performs the lubricating action while melting and absorbing the material, and this film controls the heat removal from the slab to obtain a slab with excellent surface properties and melt flux. An inexpensive mold lubricant for continuous casting and a continuous casting method, in which (molten mold lubricant) is hardly caught in the strands and clean and good cast pieces can be produced.

[0002]

2. Description of the Related Art In recent stricter quality requirements, cost reduction is also required, and in order to guarantee direct charging or direct rolling with a hot strip smoothly, only breakout and slab surface quality are required. In addition, it is necessary to manufacture a slab having no inclusions, in particular, no flux and no inclusions in the slab. In recent years, continuous casting of steel has rapidly spread because it is advantageous in terms of improving casting yield, saving resources and energy,
The continuous casting ratio of ultra low carbon steel and low carbon aluminum killed steel is already 10
It has reached 0%. In order to further improve productivity and reduce costs, and to expand and expand the advantages of the continuous casting method, as a process that expands and expands the advantage of the continuous casting method, the heating furnace charging and rolling of unmaintained hot slabs, and furthermore, the direct connection process of steelmaking and rolling, that is, direct rolling Technology has been established, and high-speed casting is indispensable for stable production and supply of high-temperature cast slabs, and some low-grade low-carbon aluminum-killed steels have been implemented.

For example, as disclosed in JP-A-61-150752, the viscosity at 1300 ° C. is 0.5 to 1.5.
Poise, a mold lubricant having a softening point of 850 to 1050 ° C and a crystallization temperature of 850 to 1050 ° C has been proposed. However, when casting with such a mold lubricant, the viscosity and crystallization temperature are very low, so the lubricity between the mold and the slab is very high, and breakout occurs in castability (productivity). Very good and less.

However, the high speed casting of 1.5 m /
min, the low viscosity of this flux,
Due to the low crystallization temperature, the molten flux is entrained in the molten steel during casting, resulting in inclusions in the slab, which directly leads to an increase in defects in the slab. Low-grade low-carbon Al
-K steel has not been particularly problematic in terms of quality. However, extremely low carbon steel, low carbon Al-K steel sheet, tin sheet, etc., in which the slab quality is strictly required, are not considered. However, a satisfactory product was not obtained, and even if casting was performed, a high yield was not obtained, and no significant productivity and cost reduction were obtained.

As for the surface quality of cast slabs, the conventional high-speed casting mold lubricant has a very low crystallization temperature, as described above, and therefore has low viscosity on the entire upper and lower inner surfaces of the mold. It is presumed to be a thin film, and therefore the heat removal from the slab is very high. For this reason,
Regarding the surface quality, even though it is an ultra-low carbon steel or low-carbon Al-K steel, cracks in the corners and surface cracks occur, causing surface flaws during hot strip maintenance-free rolling, resulting in quality deterioration. I was As described above, regarding the quality and surface quality of the slab,
He was not fully satisfied.

In order to improve the above disadvantages,
Japanese Patent Application Laid-Open No. 62-238053 discloses that as a production method free from powder-based internal defects, if the viscosity and surface tension are increased, the defects are reduced, and P = η ^−2.1 × τ.
Under the condition of ^{-0.14 W = -327 × P + 207} (P; Powder Makikomi index, eta; viscosity of powder at1300
℃, τ = surface tension of powder (dyne / cm ² ) at
1300 ° C., W; casting amount per unit time), but this merely describes the improvement of the quality of the slab. When continuous casting is performed under such conditions, it is hard to say that it has sufficient properties as a surface protective agent, and the use of a high-viscosity powder reduces the operability. It is hard to say that they were sufficiently satisfied due to the large probability of O generation, the occurrence of surface cracks due to poor lubrication and heat removal, and the like. As described above, using the current mold lubricant, at a casting speed of 0.8 m / min or more, the high-grade sheet steel can satisfy the quality (surface, internal quality) and operability, and can significantly reduce the cost and quality. There has been a demand for the development of a continuous casting mold lubricant that satisfies the improvement in steel productivity.

[0007]

SUMMARY OF THE INVENTION The present invention has been made based on the above demands. In continuous casting of ultra low carbon steel and low carbon aluminum killed steel, the turbulence of the molten steel surface increases as the casting speed increases, and a negative pressure is generated around the immersion nozzle, and the molten flux is rolled into the molten steel. Increasing the flow velocity of the molten steel on the surface increases the internal defects due to the generation of flux traps in and out of the meniscus, thereby deteriorating the quality.

Further, with the high-speed casting, the occurrence of corner cracks and vertical cracks increases due to the non-uniform generation of the solidified layer of the molten steel, and the occurrence of breakout due to the fracture of the solidified shell also increases. That is, the present invention relates to continuous casting of ultra-low carbon steel and low-carbon aluminum killed steel, and (1) reduction of winding of a mold lubricant into a slab (2) squeezing of a mold lubricant onto a slab surface; Reduction of inclusions under the epidermis (3) Production of slabs without surface cracks (4) Troubles due to sticking of the molded steel sheet and solidified shell on the molten surface in the mold (breakout and casting due to prediction of breakout) A temporary high-speed casting mold lubricant that has no surface defects (cracks, scabs, pinholes) and can obtain high-temperature, defect-free slabs with very few inclusions even in the interior. And a continuous casting method using the same.

[0009]

Means characteristic of the present invention are as follows. (1) When casting steel by continuous casting, CaO / Si
In the range of O ₂ = 0.8 to 1.3, Al ₂ O ₃ ; 2 to 10% by weight, Na ₂ O: 4% by weight or less, Li ₂ O: 4% by weight or less, and (Li ₂ O + Na ₂ O) ) Is less than 4% by weight, less than 5% by weight of aluminum fluoride and contains F ⁻ , and the carbon powder as a melting rate regulator is 0.2 to 6.0% by weight, and its physical properties are melted at 1400 ° C. When this is cooled, a mold lubricant having a crystallization rate of 10% or less and a solidification temperature of 1070 ° C. or more is used, and CaO / SiO ₂ (B), 1
The viscosity (η) at 300 ° C and the casting speed V

[0010]

(Equation 2)

[0011] A continuous casting method characterized by operating at a casting speed satisfying the relational expression: However, the viscosity (η) is Po
is at 1300 ° C, casting speed V is m / min
is there. (2) In the range of CaO / SiO ₂ = 0.8 to 1.3, A
l ₂ O ₃ ; 2 to 10% by weight, Na ₂ O: 4% by weight or less,
Li ₂ O: not more than 4% by weight and (Li ₂ O + Na ₂ O)
Is less than 4% by weight, less than 5% by weight of aluminum fluoride and contains F ⁻ , and carbon powder as a melting rate modifier is 0.2% or less.
A mold lubricant for continuous casting, characterized in that the crystallization ratio is 10% or less and the solidification temperature is 1070 ° C or more in a cooling process after its physical properties are melted at 1400 ° C.

[0012]

In general, when casting high-grade sheet steel such as ultra-low carbon steel or high-grade low-carbon aluminum killed steel at a casting speed of 0.8 m / min or more, especially 1.5 m / min or more, the mold lubricant is used. Insufficient inflow or non-uniform flow increases breakout (constraining breakout) and the incidence of casting operation troubles such as shell breakage due to a decrease in solidified shell thickness immediately below the mold. Conventionally, in order to solve this problem, a mold lubricant having a low viscosity, a low softening point and a low crystallization temperature has been used.

The present invention has developed a high-viscosity mold lubricant having new properties without using the low-viscosity metal surface protective agent as described above, especially in high-speed casting of high-grade thin steel. Various types of continuous casting operations to ensure sufficient lubrication without insufficiency in the amount of flow due to viscosity, and to obtain slag on the slab surface, no cracks, an appropriate heat removal rate, and the thickness of the solidified shell directly under the mold. Is a new continuous casting method obtained as a result of researching

In the present invention, CaO / SiO ₂ , Al ₂ O ₃ , N
Reasons for limiting the chemical composition (% by weight) of a ₂ O, Li ₂ O, Na ₂ O + Li ₂ O, and aluminum fluoride; (1) CaO
/ SiO ₂ is limited to the range of 0.8 to 1.3.
If it is less than 0.8, the viscosity becomes high, and it is difficult to adjust the physical properties. If it exceeds 1.3, it is difficult to obtain a predetermined viscosity, the glass loses its properties, and it becomes a non-lubricating mold lubricant. Is inappropriate. For this reason, CaO / SiO
₂ is limited to the range of 0.8 to 1.3.

(2) The reason why the content of Al ₂ O ₃ is limited to 2 to 10% by weight is that if the content is less than 2%, the melting temperature becomes high, and the Al ₂ O ₃ in the steel is absorbed during casting. Fluctuations in physical properties (melting point) increase. On the other hand, if it exceeds 10%, the viscosity and melting temperature become extremely high, so that the casting conditions are not satisfied, and the amount of alumina-based inclusions caused by the enrichment of Al ₂ O ₃ increases. Worsen. Therefore, the range of Al ₂ O ₃ is 2 to 10% by weight.
And

(3) Na ₂ O 4% by weight or less, Li ₂ O
The reason why the content of Li ₂ O + Na ₂ O is 4% by weight or less is (Na ₂ O or Li ₂ O or Li ₂ O + Na ₂ O).
The reason why they are 4%, 4% and 4% or less, respectively, is that when the content is more than each weight%, crystals are easily crystallized during solidification,
As a result, the unit consumption during casting decreases and the lubricity decreases.

(4) The reason that aluminum fluoride is less than 5% by weight is that aluminum fluoride is added for securing glass properties, but if added more than 5%, it becomes excessive and has no meaning. The above is unnecessary. In addition, depending on the component, predetermined physical properties may be obtained without using AlF ₃ (crystallization characteristics and solidification temperature). For this reason, the addition amount was less than 5% by weight.

(5) As the melting rate adjusting agent, the carbonaceous powder needs to be 0.2 to 6.0. If it is less than 0.2%, in the case of the target steel type, the melting rate is too high in any case, and it is difficult to adjust. On the other hand, if it exceeds 6%, the thickness of the molten pool becomes thin and outside the appropriate range of the thickness of the molten pool. Therefore, the carbon powder is optimally 0.2 to 6.0%.

(6) Powder composition (CaO / SiO
₂ ), the relationship between the viscosity at 1300 ° C., and the casting speed must satisfy η> 3 / (B × V). This is Vc
= 0.8 m / min, B is at least 0.8 or more and η
Requires 4.7 poise or more, and Vc = 2.
In the case of 5 m / min, B is 0.9 or more and η is 1.4 po.
more than is required.

(7) Next, as the characteristics of the powder,
When the molten flux at 400 ° C. is measured by a measuring method as shown in FIG. 2, the crystallization ratio needs to be 10% or less, and the solidification temperature needs to be 1070 ° C. or more. This is a physical property obtained within the range of the chemical composition of the above (1) to (4), and this condition is an essential condition while satisfying the range of the component composition. The reason for this is that a mold lubricant that causes crystallization during solidification by slow cooling causes poor lubrication on the low temperature side immediately below the meniscus in the mold or makes it impossible to flow uniformly. For this reason, at the time of solidification, the crystallization ratio needs to be 10% or less.

(8) The solidification temperature when the molten flux solidifies must be 1070 ° C. or higher. This is because if the solidification temperature is less than 1070 ° C. under the same conditions as in (7), the slag film in the mold has a strong vitreous property, the film thickness is reduced, and the slag film is strongly cooled in the mold. It causes surface cracks and causes uneven and strong cooling. For this reason, the solidification temperature needs to be 1070 ° C. or higher.

(9) Further, an appropriate amount of fluorine F ^- is added as a physical property modifier. Fluorine F ^- is CaF ₂ , NaF, Li
F or the like. These (1)-
By satisfying the condition (9), it is possible to produce an excellent cast piece under continuous casting. The mold lubricant according to the present invention may be any raw material system prepared using an amorphous or amorphous base material, or using a crystalline base material such as Portland cement, wollastenite, or dicalcium silicate. There is no problem even if it is a powder, granule, hollow or spherical as long as the conditions are satisfied.

[0023]

Next, Table 1 shows the mold lubricant for continuous casting of the present invention, the continuous casting method, and the results of casting by the conventional method. Using each mold lubricant shown in Table 1, continuous casting was performed at various casting speeds (0.8 to 2.7 m / min). Table 2 shows the casting results in that case. The present invention shown in Tables 1 and 2 and the contents and evaluation under the conditions are shown. Table 1
A to h satisfy the conditions satisfying the present invention, and the material and form of the mold lubricant are not limited. Good results were shown in both the quality and operability of the slab.

[0024]

[Table 1]

[0025]

[Table 2]

I to p are comparative examples (conventional examples). Although i is within the range of the present invention in terms of chemical composition and viscosity, and casting conditions, the crystallinity is large and not satisfactory. Although the quality of the slab was good, the sticking occurrence index was large. As for j, Na ₂ O and Na ₂ O + Li ₂ O were out of the range of the present invention in the chemical composition, and as a result, crystallization occurred upon cooling. . As for k, since Al ₂ O ₃ is outside the range of the present invention and Na ₂ O + Li ₂ O is at the upper limit outside the range, crystallization is large, and as a result, i, j
Similar to the above, good results cannot be obtained for operability.

1 indicates that the content of AlF ₃ is out of the range of the present invention. Therefore, there is no crystallization. However, the solidification temperature is 1050 ° C., which is outside the range of the present invention. For that reason, the cooling becomes strong and the surface crack occurs. m is a component other than carbon powder that satisfies the present invention, but is as small as 0.1% carbon powder. As a result, the thickness of the molten slag pool becomes excessive and not only the operability is reduced but also the quality is good. Nothing was obtained. n satisfies the chemical composition but Ca
When O / SiO ₂ is too high, crystallization becomes large upon cooling. As a result, the operability was reduced.

In the case of o, CaO / SiO ₂ was out of the lower range. As a result, the solidification temperature was lowered to 1000 ° C., and there was no problem in operability, but the occurrence of surface cracks was large.
g ′ and e ′ are both above CaO / SiO ₂ of the present invention.
In the case of g ', slacking BO and surface cracks occur due to poor lubrication. In the case of g', e 'is not shown in the table. -Good results were not obtained for both the internal and external quality.

P did not satisfy η> 3 / (B × V), and the viscosity was too low and the internal defect was large.

When the solidification temperature and viscosity are measured, they are measured by a rotating cylinder method. The temperature is gradually lowered from 1300 ° C., and the temperature at which the viscosity rises rapidly is defined as the solidification temperature (see FIG. 1). . The method of measuring the crystallinity of the flux melted at 1400 ° C. was measured by the method shown in FIG.

In the measurement example in FIG. 1, 200 gr of various mold lubricants after decarburization were melted at 1400 ° C. for 10 minutes,
After maintaining the temperature at 0 ° C., the viscosity was measured. Then, the temperature was lowered by 20 ° C., and the viscosity was measured after maintaining at each temperature, and log η and 1 / T (η indicates the viscosity poise, and T indicates the absolute temperature K) ) Created a graph of the relationship. At the time of this measurement, a crystal was crystallized in the sample due to a temperature drop and became a Newtonian fluid, and a temperature 10 ° C. higher than the temperature at which measurement became impossible was defined as a break point.

The steps of the method of measuring the crystallinity of the flux shown in FIGS. 2A to 2D are as follows. 1) 110 g of the decarburized sample is put into the graphite crucible (I) and put into the graphite crucible (II). 2) Hold and melt at 1400 ° C for 10 minutes in a muffle furnace. 3) Raise the graphite crucible (I) and cast the melted sample into a V-shaped frame. 4) After air cooling, remove the mold. 5) The die-cut sample is cut at the center to evaluate the crystallization ratio. The crystallization ratio was calculated by the following equation based on (d) a sample for evaluating the crystallization ratio.

[0033]

(Equation 3)

[0034]

According to the present invention, with the advancement of the continuous casting technology of steel, high speed and no maintenance are aimed at, and at the same time, the quality of the slab is also upgraded, and the conventional low speed casting is aimed at. What has been required, a continuous casting method of extremely useful ultra-low carbon steel and high-grade low carbon Al-K steel which enables the production of good cast slabs even at high speed casting conditions has been advantageously enabled. Things.

[Brief description of the drawings]

FIG. 1 is a graph showing a method for determining a solidification temperature and a viscosity of a flux (mold lubricant for continuous casting).

FIG. 2 is an explanatory view showing a process procedure of a method for measuring a crystal ratio of a flux. (B) Cast into a V-shaped frame. (B) Remove the mold. (C) Central cutting and observation (d) Evaluation of crystallization rate

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Taku Yoshimitsu 2544-6 Oya Hachiya, Buzen-shi, Fukuoka Nippon Steel Construction Materials Industry Co., Ltd. (72) Inventor Akira Matsuo 2544-6 Oya Hachiya, Buzen-shi, Fukuoka (56) References JP-A-61-150752 (JP, A) JP-A-2-1655853 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/108 B22D 11/07

Claims (2)

(57) [Claims] 1. When casting steel by continuous casting, Ca
O / SiO ₂ = 0.8 to 1.3, Al ₂ O ₃ ;
2 to 10% by weight, Na ₂ O; 4% by weight or less, Li ₂ O;
4% by weight or less and (Li ₂ O + Na ₂ O) 4% by weight
Hereinafter, when the aluminum powder contains less than 5% by weight of aluminum fluoride and contains F ⁻ , the carbon powder as a melting rate regulator is 0.2 to 6.0% by weight, and its physical properties are melted at 1400 ° C. and then cooled. Crystallization rate of 10% or less and solidification temperature of 1070
Use a mold lubricant having a temperature of
O ₂ (B), viscosity (η) at 1300 ° C., and casting speed V are as follows: A continuous casting method characterized by operating at a casting speed satisfying the relational expression: However, the viscosity (η) is Poise at
At 1300 ° C., the casting speed V is m / min. 2. In a range of CaO / SiO ₂ = 0.8 to 1.3, Al ₂ O ₃ ; 2 to 10% by weight, Na ₂ O: 4% by weight.
Below, Li ₂ O; not more than 4% by weight and (Li ₂ O + Na ₂
O) is 4% by weight or less, less than 5% by weight of aluminum fluoride and contains F ⁻ , the carbon powder as a melting rate regulator is 0.2 to 6.0% by weight, and its physical properties melt at 1400 ° C. A mold lubricant for continuous casting, wherein a crystallization ratio is 10% or less and a solidification temperature is 1070 ° C. or more in a cooling process after the cooling.