JPH10263767A - Continuous casting method of ultra low carbon steel and mold powder for continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably obtain a good cast slab having a little development of defect in a cold-rolling process so as not to develop pickup of the carbon in a continuous casting by specifying casting speed and using mold powder specifying the basicity, viscosity and solidified temp. SOLUTION: The casting speed is made to 1.4-2.8 m/min. Further, as the mold powder, the one having >=0.85 basicity, 3-15 poise viscosity at 1300 deg.C and >=1000 deg.C solidified temp., desirably, 1050-1120 deg.C, is used. This mold powder consisting essentially of CaO, SiO2 and Al2 O3 , is profitable. Since the solidified temp. of the mold powder is made to >=1000 deg.C, the thickness of slag rim in a mold becomes suitable thickness and non-solidified powder in the solid phase powder is prevented from flow-in between the mold and the cast slab. Further, in the case the solidified temp. of the mold powder is >=1000 deg.C, the increasing of alarming frequency of the breakout as a trouble is not developed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold powder for continuous casting, and more particularly to a mold powder suitable for use in continuous casting of ultra-low carbon steel for use in steel plates for automobiles, steel plates for cans and the like, and this mold powder. And a continuous casting method using the same.

[0002]

2. Description of the Related Art Mold powder for continuous casting of steel is added onto the surface of molten steel poured into a mold, and mainly comprises:
(1) Lubrication between the mold and the solidified shell, (2) Absorption and removal of inclusions in the steel that floated, (3) Heat retention of molten steel, and (4) Antioxidation of molten steel. Such mold powder,
It is an important auxiliary material in the continuous casting process.
Based on CaO, Al ₂ O ₃ , SiO ₂ , and Na ₂ O,
NaF, CaF ₂ , MgO, etc. are blended to adjust the viscosity, softening temperature, and solidification temperature of the powder according to the casting speed of the target steel slab.

[0003] In the continuous casting operation, however, due to local fluctuations in the molten metal level due to the operating conditions or changes in the operating conditions.
The molten powder may get caught in the molten steel injection flow into the mold, or adhere to the solidified shell, and if the rolling process is performed in such a state, it will be extended and cause surface defects of the cold-rolled steel sheet, The powder for continuous casting is required not to cause such surface defects.

[0004] Such a cold-rolled steel sheet derived from powder
Powder for continuous casting to prevent surface defects
For example, Japanese Patent Application Laid-Open No. 8-33962 discloses
There are things that are. That is, the model disclosed in this gazette
Powder is CaO-Al_TwoO _Three−SiO_TwoZr
O_Two, TiO_Two, Cr_TwoO_ThreeOne or more oxides in total of 8 to
Contains 15wt%, MnO, FeO, Fe_TwoO_ThreeAnd CaO
One or more oxides containing a total of 5 to 12 wt%
In particular, as mold powder for ultra-low carbon steel,
Solid temperature below 1000 ℃, viscosity at 1300 ℃ is 1.5 po
It is additionally stipulated that ise or more.

The reason why the viscosity is increased as described above is that the extremely low carbon steel tends to develop abnormally at the claw portion at the tip of the solidified shell during casting, and it is intended to minimize the problem of powder entrainment resulting therefrom. The reason for lowering the solidification temperature is to prevent shortage of powder consumption and to avoid an increase in the frequency of breakout alarms.

[0006]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No. 8-339.
In the case of the mold powder for ultra-low carbon steel disclosed in Japanese Patent No. 62, since the solidification temperature is low even when the viscosity is increased, the slag rim in the mold (solidified powder that develops at the boundary between the powder molten layer and the mold) The unmelted powder flows from the solid phase powder layer between the mold and the slab during casting, and the unmelted powder is cast into the slab by the foot roll of the mold and the support roll of the secondary cooling zone. By being pressed, a region (carbon pickup) having a high C concentration is locally generated in the surface layer portion of the slab.

[0007] In the case of a low carbon steel sheet having a C content of 0.01 wt% or more, even if such a local carbon pickup is present, the properties of the steel sheet do not change significantly, and there is no particular problem. However, in a very low carbon steel sheet having a C content of less than 0.01 wt%, the carbon concentration of the steel sheet itself is extremely low, so that the properties are greatly changed by the carbon pickup. One of them is that the hardness of a steel sheet is locally large, so that the formability in press forming or the like is locally deteriorated. Second, since the activity of the steel sheet surface is locally changed, unevenness occurs during surface treatment such as chromate treatment, phosphate conversion treatment, electrogalvanizing, and hot-dip galvanizing. When an ultra-low carbon steel sheet is used as a steel sheet for automobiles or a steel sheet for cans, formability and surface treatment properties are particularly important, and it is therefore desired to prevent the carbon pickup as much as possible.

The present invention advantageously solves the above-mentioned problems, and minimizes the occurrence of carbon pick-up during continuous casting, thereby making it suitable for use as a steel plate for automobiles and steel plates for cans. It is an object of the present invention to propose a mold powder with an advantageous continuous casting method using the mold powder.

[0009]

The continuous casting method for ultra-low carbon steel according to the present invention provides a method for continuously casting ultra-low carbon steel at a casting speed of 1.4 to 2.8 m / min and a basicity as a mold powder. Is 0.85 or more and the viscosity at 1300 ° C is 3 to 15
It is characterized by using a poise having a solidification temperature of 1000 ° C. or higher. In such a continuous casting method, the solidification temperature of the mold powder is 1050 ℃ ~ 1120 ℃,
More advantageously fit. Further, the continuous casting mold powder of very low carbon steel of the present invention, CaO, as main components SiO ₂ and Al ₂ O _3, basicity least 0.85, a viscosity at 1300 ° C. is 3～15Poise, solidification temperature is 1000 C. or higher. In such a mold powder for continuous casting, it is more advantageous that the solidification temperature is from 1050 ° C to 1120 ° C.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The mold powder of the present invention
Since the solidification temperature of the mold powder is 1000 ° C. or higher, the thickness of the slag rim in the mold becomes an appropriate thickness, so that unsolidified powder in the powder solid layer can be prevented from flowing into the space between the mold and the slab. become. Therefore, there is no carbon pickup and no problems caused by it. A more preferred solidification temperature is between 1050 and 1120 ° C. That is, when the temperature is higher than 1120 ° C., the slag rim is easily developed, and the portion called the claw at the tip of the solidified shell falls down into the oscillation portion of the mold, so that inclusions are easily captured. This is because the surface defects increase and the thickness of the slag rim becomes more appropriate at 1050 ° C. or higher.

[0011] It should be noted that an increase in the frequency of breakout alarms, which is a concern when the solidification temperature is set to 1000 ° C or higher, does not occur. Because, as shown in FIG. 3, the relationship between the solidification temperature of the mold powder and the breakout alarm occurrence ratio index is shown in FIG.
This is because the frequency of the breakout alarm increases when the temperature is 1200 ° C. or more, and when the temperature is 1050 to 1120 ° C., there is no problem.

Next, in the present invention, the viscosity of the mold powder at 1300 ° C. is set to 3 to 15 poise for the following reason. In high-speed continuous casting of steel performed for the purpose of high productivity, the flow rate of molten steel in a mold is increased due to a large flow rate of molten steel injected from a tundish into a mold. Particularly, the flow of the molten steel that collides with the short side of the mold and goes upward disturbs the interface between the molten powder on the meniscus and the molten steel, entrains the powder into the molten steel, and causes norokami defects in the slab. This tendency increases as the melt viscosity of the mold powder decreases. Therefore, in the present invention, by setting the viscosity of the mold powder to 3 poise or more, the above-mentioned entrapment of the mold powder can be prevented even at a casting speed of 1.4 to 2.8 m / min which is a high speed casting range. However, if the viscosity exceeds 15 poise, it becomes impossible to properly flow the powder into the gap between the mold and the solidified shell, causing poor lubrication between the slab and the mold, causing serious operational troubles such as breakout. Therefore, the viscosity of the mold powder is set to 3 to 15 poise.

The reason for setting the basicity of the mold powder to 0.85 or more is that by setting the basicity to 0.85 or more, a high absorption ability for deoxidation products such as alumina and TiO ₂ present in molten steel can be obtained, and The reason is that even if these deoxidized products are absorbed to some extent, the viscosity and the coagulation temperature are stable without changing much.

[0014] The mold powder for continuous casting of the present invention contains CaO, SiO ₂ and Al ₂ O ₃ as main components.
In order to adjust the solidification temperature and viscosity, for example, B ₂ O ₃ , MgO
, F ₂ , Na ₂ O, Li ₂ O, etc. are added at about several percent. Al ₂
BaO or the like may be added to improve O ₃ absorption capacity. The amounts of these components added may be arbitrarily selected depending on the purpose within a range satisfying the requirements specified in the present invention. In addition, these additives are contained in the conventional mold powder.

[0015]

EXAMPLES Various mold powders having the components and properties shown in Table 1 were prepared. Sample A is a comparative example having a high solidification temperature, sample B is an example, and sample C is a conventional example having a low viscosity. These mold powders were converted to ultra-low carbon steel (C: 0.00
15wt%) (casting speed: 1.6m / min, maximum casting speed 1.7m / min), the presence or absence of carbon pick-up and, if it occurs, the results of investigation of the carbon concentration in that part and the result of investigation of the slag rim thickness It is shown in FIG.

[0016]

[Table 1]

[0017]

[Table 2]

As is apparent from Table 2, Examples B _{1 to} B
_In No. ₄ , the slag rim thickness was larger than that of the conventional example C, and as a result, the carbon pickup was eliminated. FIG. 1 shows the ratio of the surface defect of the cold-rolled coil by powder. Example B ₁ .about.B ₄ good results that defect ratio of the coil is low is obtained. Comparative Example A and Example B ₁
When comparing .about.B _4, Example B, the thin rim thickness compared to Comparative Example A, the nail depth is also shallow as shown in FIG. As a result, as shown in FIG. 1, a favorable result that the sample B had a low coil defect ratio was obtained.

[0019]

By using the mold powder of the present invention, no carbon pick-up occurs even when ultra-low carbon steel is cast at a high speed, so that a good cast piece with few defects in the cold rolling process can be obtained. Casting can be performed under stable conditions.

[Brief description of the drawings]

FIG. 1 is a view showing a surface defect occurrence index of a coil for each sample.

FIG. 2 is a diagram illustrating a relationship between a slag rim thickness and a claw depth.

FIG. 3 is a diagram showing a relationship between a solidification temperature of mold powder and a breakout alarm occurrence ratio index.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Akira Yamauchi 1st Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Chiba Works of Kawasaki Steel Co., Ltd. No. 26 Sakai Chemical Industry Co., Ltd.

Claims (4)

[Claims] 1. The casting speed during continuous casting of ultra-low carbon steel
1.4 to 2.8 m / min and mold powder with basicity of 0.85 or more and viscosity at 1300 ° C of 3 to 15 po
A continuous casting method for ultra-low carbon steel, wherein ise and a solidification temperature of 1000 ° C. or higher are used. 2. The solidification temperature of the mold powder is 1050 ° C. or more.
The continuous casting method for ultra low carbon steel according to claim 1, wherein the temperature is 1120 ° C. 3. A composition comprising CaO, SiO ₂ and Al ₂ O ₃ as main components, a basicity of 0.85 or more, and a viscosity at 1300 ° C. of 3 to 15 poise.
Mold powder for continuous casting of ultra-low carbon steel, having a solidification temperature of 1000 ° C or higher. 4. The solidification temperature of the mold powder is 1050 ° C. or more.
The mold powder for continuous casting of ultra-low carbon steel according to claim 3, which is at 1120 ° C.