JP2001205401A - Mold powder for continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide mold powder with which the occurrence of breakout may be prevented and a slab having good surface quality may be obtained without the occurrence of longitudinal cracking. SOLUTION: The mold powder consisting of CaAO, SiO2 and fluorine compound as a basic composition is <=20 deg.C in the absolute value of the fluctuation in solidifying point with respect to a fluctuation of 1 mass% of the content of Al2O3, MnO or F or a fluctuation of 0.1 of the ratio Ca/SiO2 of mass% of CaO to a SiO2 and is <=1 poise in the absolute value of the fluctuation in viscosity at 1,300 deg.C.

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 of steel capable of preventing the occurrence of breakout and the occurrence of vertical cracks on the surface of a slab.

[0002]

2. Description of the Related Art In a continuous casting method of steel, molten steel is supplied into a mold through an immersion nozzle, and mold powder is injected into the surface of the molten steel. The mold powder is usually composed of powders or granules of a plurality of types of oxides or carbon. The mold powder put into the mold is melted by the heat of the molten steel and becomes molten slag. This molten slag flows into the gap between the inner wall of the mold and the solidified shell, and a slag film is formed.

[0003] The mold powder exhibiting such behavior has the following effects. 1) Keeping the temperature of the molten steel in the mold and preventing oxidation of the molten steel 2) Ensuring lubricity between the inner wall of the mold and the solidified shell 3) Absorbing bubbles and oxides floating on the surface of the molten steel in the mold 4) Mold Adjusting the cooling rate of the solidified shell by adjusting the heat transfer between the inner wall and the solidified shell.

[0004] In the recent continuous casting method of steel, high-speed casting of about 2 m / min or more is performed from the viewpoint of securing productivity and reducing manufacturing costs. Therefore, breakout is likely to occur due to poor lubrication between the inner wall of the mold and the solidified shell. In addition, with the expansion of the application of continuous casting to various products, the casting of hypoperitectic steel is increasing. In this hypoperitectic steel, longitudinal cracks are likely to occur on the surface of the slab due to its solidification characteristics.

[0005] In the role of the mold powder described in 1) to 4) above, to prevent the occurrence of breakout,
Adjustment of the cooling rate of the solidified shell shown in 4) is important for ensuring lubricity shown in 2) and preventing longitudinal cracks on the slab surface.

Therefore, for the purpose of preventing the occurrence of breakout and the occurrence of vertical cracks on the surface of the slab, the chemical composition of the mold powder and the physical properties such as solidification point and viscosity have been conventionally adjusted.

[0007] For example, in Japanese Patent Application Laid-Open No. 10-314897, the basicity (weight ratio of CaO / SiO ₂ ) and A
Mold powder has been proposed which can prevent the occurrence of longitudinal cracks on the surface of a slab such as hypoperitectic steel by defining the respective contents of l ₂ O ₃ and F.

In Japanese Patent Application Laid-Open No. Hei 10-263767, the basicity, the freezing point and the viscosity are defined by
There has been proposed a mold powder capable of preventing the occurrence of breakout when casting ultra-low carbon steel.

[0009] However, the above-mentioned Japanese Patent Application Laid-Open No. 10-314897.
However, depending on the casting conditions, vertical cracks may occur on the surface of the slab of subperitectic steel, or high-speed casting of ultra-low carbon steel may be performed using the mold powder proposed in Japanese Patent Application Laid-Open No. 10-263767 or JP-A-10-263767. In some cases, a breakout may occur.

[0010]

DISCLOSURE OF THE INVENTION The present invention provides a continuous casting method capable of preventing the occurrence of breakout during high-speed casting and preventing the occurrence of longitudinal cracks on the surface of a slab when casting hypoperitectic steel. An object of the present invention is to provide a mold powder for use.

[0011]

The gist of the present invention is as follows.
For mold powder for continuous casting of steel shown in (1) to (4)
is there. (1) CaO, SiO_Two And fluorine compound
In the mold powder,_Two O_Three , MnO also
Or 1% by mass variation of the F content or SiO_Two To
Ratio of mass% of CaO to CaO / SiO_Two Of 0.1
The absolute value of the freezing point fluctuation is less than 20 ° C
And the absolute value of the change in viscosity at 1300 ° C is 1 poise or less.
Below is the mold powder for continuous casting of steel. (2) SiO of CaO_Two Ratio of mass% to CaO / S
iO_Two Is 0.6 to 1.5 and in mass%, CaO: 1
0-60%, SiO_Two : 10 to 65%, Al_Two O _Three : 1
-5%, F: 2-20%, MgO, B_Two O_Three You
And ZrO_Two Total content of 1-20%, Na_Two CO
_Three And Li_Two CO_Three Is 2 to 20%.
A mold powder for continuous casting of steel according to the above (1). (3) SiO of CaO_Two Ratio of mass% to CaO / S
iO_Two Is 1.8 to 4.0 and in mass%, CaO: 1
0-70%, SiO_Two : 10 to 30%, Al_Two O _Three : 5
-20%, F: 5-30%, MgO, B_Two O_Three 
And ZrO_Two Total content of 1 to 25%, Na_Two C
O_Three And Li_Two CO_Three Total content of 2-25%
A mold pow for continuous casting of steel according to (1) above.
Da. (4) SiO of CaO_Two Ratio of mass% to CaO / S
iO_Two Is 5.5 to 7.0 and in mass%, CaO: 1
0-70%, SiO_Two : 5 to 15%, Al_Two O_Three: 10
-40%, F: 10-30%, MgO, B_Two O
_Three And ZrO_TwoTotal content of 1 to 30%, Na_Two 
CO_Three And Li_Two CO_Three Total content of 2 to 30%
The mold pow for continuous casting of steel according to (1) above,
Da.

The present inventors have solved the above-mentioned problems of the present invention by taking measures based on the following findings.

For the purpose of preventing the occurrence of breakout and the occurrence of vertical cracks on the slab surface, the chemical composition, solidification point,
Mold powder in which physical properties such as viscosity are adjusted is used. However, at present, the problems of breakout during casting and vertical cracks on the slab surface have not been completely solved.

[0014] Even when using mold powder whose chemical composition and physical properties are adjusted, breakouts and vertical cracks on the slab surface are caused by the solidification point of molten slag (melted mold powder) and It was found that the physical property value of the viscosity fluctuated during casting.

The solidification point and viscosity of the molten slag fluctuate during casting because the chemical composition of the molten slag fluctuates during casting. The chemical composition of the molten slag fluctuates during casting for the following reason. That is, due to the reaction between the molten steel in the mold and the molten slag, the contents of SiO ₂ , Al ₂ O ₃ , MnO, and the like in the molten slag fluctuate. Further, since F in the mold powder easily reacts with moisture in the mold powder, the F content in the molten slag may decrease.

When the content of Al ₂ O ₃ , MnO, and F in the molten slag fluctuates, or when the content of SiO ₂ fluctuates, the ratio of the mass% of CaO to SiO ₂ CaO / When the value of SiO ₂ fluctuates, the physical properties of the solidification point and viscosity of the molten slag tend to fluctuate.

If the solidification point and the physical properties of the viscosity of the molten slag fluctuate and become higher, the molten slag is less likely to flow into the gap between the inner wall of the mold and the solidified shell, so that breakout tends to occur. Further, when the physical properties of the solidification point and the viscosity fluctuate and decrease, the heat transfer between the inner wall of the mold and the solidified shell increases, so that vertical cracks are easily generated on the surface of the slab.

The mold powder of the present invention comprises CaO, S
The iO ₂ and fluorine compound in mold powder of a basic composition, Al ₂ O _3, 1 wt% of the variation in the content of MnO or F, or the mass% of CaO with respect to SiO ₂ ratio CaO / SiO ₂ 0.1 The mold powder has an absolute value of a change in freezing point of 20 ° C. or less and an absolute value of a change in viscosity at 1300 ° C. of 1 poise or less. In such a mold powder, fluctuations in the solidification point and viscosity of the molten slag during casting are small, so that breakout and vertical cracks on the slab surface are prevented.

The mold powder having such characteristics is a mold powder having the chemical composition described in the above (2) to (4).

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION When the absolute value of the change in the freezing point exceeds 20 ° C. and when the absolute value of the change in the viscosity at 1300 ° C. exceeds 1 poise, the range of these changes is too large. Breakout is likely to occur, and vertical cracks are likely to occur on the slab surface.

[0021] The mold powder of the present invention is a mold powder in which fluctuations in the freezing point and viscosity are reduced even if the chemical composition of the molten slag fluctuates during casting. Specifically, A
1% by mass variation in the content of l ₂ O ₃ , MnO or F, or the ratio of the mass% of CaO to SiO ₂ CaO
The absolute value of the change in the freezing point is 20 ° C. or less and the absolute value of the change in the viscosity at 1300 ° C. is 1 poise or less with respect to the change of 0.1 in / SiO ₂ .

The mold powder of the present invention having such characteristics can be classified into the mold powder having the chemical composition shown in the above (2) to (4), if it is classified in the range of the value of the mass% of CaO / SiO _2. is there.

The mold powder having such a chemical composition is a mold powder in which a liquid phase can be stably present since the gradient of the composition-temperature of the liquidus line is small in the chemical composition-temperature phase diagram of the mold powder. is there. In other words, even if the chemical composition of the molten slag slightly fluctuates, the liquid phase is stably present and the composition-temperature gradient of the liquidus line is small,
Fluctuations in the freezing point and viscosity of the molten slag are small.

When manufacturing the mold powder of the present invention,
The raw materials used are commonly used mold powders.
It does not matter. As the CaO raw material, quick lime, lime
Stone, cement, SiO_Two Raw materials include silica sand, light algal soil,
Al_Two O_Three As raw materials, alumina powder, raw material for F
Fluorite, sodium fluoride, MgO
O clinker, magnesium carbonate, B_Two O_Three As raw material
Is borax, ZrO_Two Raw materials include zircon sand, N
a_Two CO_Three Raw materials include soda ash and Li_Two CO _Three material
For example, lithium carbonate or the like may be used.

Further, the particle size of the raw material is desirably 100 μm or less.
New In addition, these raw materials include Fe _Two O_Three , Fe_Three O_Four 
Oxides, etc.
It will be included inevitably. But these impurities
Even if there is, there is no particular problem.

[0026]

EXAMPLE A vertical bending type continuous casting machine having a vertical portion of 1.5 m and a radius of curvature of 3.5 m was used. The thickness was 90 mm and the width was 1200 mm.
Was cast. Table 1 shows the chemical composition of the cast steel. The casting speed was 3.5 m / min for the low carbon steel of steel A and 2.5 m / min for the hypoperitectic steel of steel B. In each test, 1
About 150 tons of heat were cast.

[0027]

[Table 1]

Table 2 shows the chemical composition of the mold powder used in the test. Table 3 Al ₂ O _3, MnO, when the respective content of F is varied to increase by 1 wt%, or the wt% of CaO with respect to SiO ₂ ratio CaO / SiO ₂
Shows the amount of change in the solidification point and the amount of change in the viscosity of each molten slag when the value fluctuates by increasing by 0.1. When the freezing point and the viscosity decrease or decrease, respectively, the freezing point fluctuation amount and the viscosity fluctuation amount are indicated with a minus sign. When the freezing point and the viscosity increase or increase respectively, only numerical values are displayed.

[0029]

[Table 2]

[0030]

[Table 3]

During casting, the occurrence of breakout was investigated. Further, the molten slag in the mold was collected and analyzed for its chemical composition. In addition, in each test, 10
A m-length slab was sampled, the slab surface was visually observed, and the occurrence of longitudinal cracks was evaluated. In the evaluation A, a vertical crack was not generated or only a very small number was generated.
Has about 10 vertical cracks having a length of about 20 to 100 mm, and the depth C of the vertical cracks is shallow.
Deep vertical cracking or length 200mm
It is assumed that vertical cracks of a degree or more are extremely large. Table 4 shows test conditions and test results.

[0032]

[Table 4]

Test No. of the present invention example Test No. 1 and Comparative Example No. 2-No. In 5, the low carbon steel of steel A is 3.5m
/ Min speed. The value of the ratio CaO / SiO ₂ of the mold powder used in each test is 1.0. Test No. In No. 1, a mold powder a1 was used in which the amount of change in the freezing point and the amount of change in the viscosity when the chemical composition of the molten slag fluctuated were within the ranges defined by the present invention. Test No. 2-No. In No. 5, the mold powder Al ₂ O
_{Because of the high} content of ₃ , F, (sum of the contents of MgO, B ₂ O ₃ and ZrO ₂ ), or (sum of the contents of Na ₂ CO ₃ and Li ₂ CO ₃ ), the chemistry of the molten slag is high. Mold powders a2 to a5 having a large freezing point fluctuation and a high viscosity fluctuation when the composition fluctuates are out of the range defined by the present invention.

Test No. In No. 1, no vertical cracks occurred on the slab surface, and no breakout occurred. From the beginning of casting, the viscosity and solidification point of the mold powder are high,
Test No. 2 using mold powder a2 having a large variation in viscosity and a large variation in freezing point when the chemical composition of the molten slag varied. In No. 2, a breakout occurred during casting. Before the breakout occurred, the surface of the slab had a slight vertical crack of evaluation B. Test Nos. Using mold powders a3 to a5 having low viscosity and solidification point of the mold powder from the beginning of casting, and large fluctuations in the solidification point and viscosity when the chemical composition of the molten slag fluctuated. Three
-No. In No. 5, a remarkable vertical crack of evaluation C occurred on the slab surface.

Test No. of the present invention example Test No. 6 and Comparative Example No. 7-No. In No. 10, the low carbon steel of steel A was 3.5
Casting was performed at a speed of m / min. The value of the mold powder ratio CaO / SiO ₂ used in each test is 3.2.
Test No. In No. 6, mold powder b1 was used in which the solidification point variation and the viscosity variation when the chemical composition of the molten slag fluctuated were within the ranges defined by the present invention. Test No. 7-No. In No. 10, the mold powder Al ₂
Due to the high content of O ₃ , F, (sum of the contents of MgO, B ₂ O ₃ and ZrO ₂ ), or (sum of the contents of Na ₂ CO ₃ and Li ₂ CO ₃ ), Mold powders b2 to b5 having large freezing point fluctuations and viscosity fluctuations when the chemical composition fluctuates are out of the range defined by the present invention.

Test No. In No. 6, no vertical cracks occurred on the slab surface, and no breakout occurred. Test N using mold powder b2 which has a slightly higher viscosity and a higher freezing point when the chemical composition of the molten slag fluctuates due to a slightly higher viscosity and freezing point of the powder from the beginning of casting.
o. In No. 7, a breakout occurred during casting. Before the breakout occurred, the surface of the slab had a slight vertical crack of evaluation B. Mold powders b3 to b having a low solidification point of the mold powder and a large solidification point variation and a large viscosity variation when the chemical composition of the molten slag varies.
Test No. 5 using No. 5 8 to No. In No. 10, a remarkable vertical crack of evaluation C occurred on the slab surface.

Test No. of the present invention example Test No. 11 and Comparative Example No. 12-No. In No. 15, low carbon steel of steel A was cast at a speed of 3.5 m / min. The value of the mold powder ratio CaO / SiO ₂ used in each test is 6.2. Test No. In No. 11, mold powder c1 was used in which the solidification point fluctuation amount and the viscosity fluctuation amount when the chemical composition of the molten slag fluctuated were within the conditions specified in the present invention. Test No. In No. 12, the mold powder Al ₂ O
Test No. ₃ due to the low content of Test No. 3 13-No.
In No. 15, F, (sum of the contents of MgO, B ₂ O ₃ and ZrO ₂ ) of the mold powder, or (Na ₂ CO ₃
And the total content of Li ₂ CO ₃ ), the solidification point variation and the viscosity variation when the chemical composition of the molten slag fluctuates are large outside the conditions specified in the present invention. Powders c2 to c5 were used, respectively.

Test No. In No. 11, no vertical cracks occurred on the slab surface, and no breakout occurred.
Test No. 2 using mold powders c2 to c5 having a large variation in viscosity and a large variation in solidification point when the chemical composition of the molten slag varies. 12-No. In No. 15, a remarkable vertical crack of evaluation C occurred on the slab surface.

Test No. of Comparative Example 16 and No. 17
Then, the low carbon steel of the steel A was produced at a speed of 3.5 m / min at a specific Ca
Casting was performed using mold powders having O / SiO ₂ values of 1.6 and 8.0. Mold powder d whose solidification point is low from the beginning of casting and whose solidification point fluctuation and viscosity fluctuation are large when the chemical composition of the molten slag changes.
Test No. In No. 16, a remarkable vertical crack of evaluation C occurred on the slab surface. Test No. 3 using mold powder e having a high viscosity fluctuation amount and a large freezing point fluctuation amount when the chemical composition of the molten slag fluctuates when the viscosity of the mold powder is high from the beginning of casting. In No. 17, a breakout occurred at the beginning of casting.

Test No. of the present invention example Test No. 18 and Comparative Example No. In No. 19, a hypoperitectic steel of steel B was cast at a speed of 2.5 m / min. Test No. 1 using the mold powder a1 described above. In No. 18, no vertical cracks occurred on the slab surface, and no breakout occurred. Test No. 1 using mold powder e described above. 19, the casting speed is 2.
Since it was 5 m / min, no breakout occurred. However, since the amount of change in the viscosity and the amount of freezing point change when the chemical composition of the molten slag changes, the specific CaO
Despite the use of mold powder having a large value of / SiO ₂ , a remarkable vertical crack of evaluation C occurred on the slab surface.

[0041]

By applying the mold powder of the present invention, the occurrence of breakout during high-speed casting can be prevented, so that stable operation is possible. Also, when casting hypoperitectic steel, it is possible to prevent the occurrence of vertical cracks on the slab surface,
A slab with good surface quality can be obtained.

Claims (4)

[Claims] 1. A mold powder having a basic composition of CaO, SiO ₂ and a fluorine compound, wherein Al ₂ O ₃ , M
1% by mass variation of the content of nO or F, or Si
The ratio of the mass% of CaO to O _2, the absolute value of the variation of the freezing point is 20 ° C. for a variation of 0.1 of CaO / SiO _2.
And the absolute value of the change in viscosity at 1300 ° C. is 1 po
a mold powder for continuous casting of steel, wherein the powder is not more than "ise". 2. The ratio of the mass% of CaO to SiO ₂ Ca
O / SiO ₂ is 0.6 to 1.5 and in mass%, Ca
_{O: 10~60%, SiO 2:} 10~65%, Al 2 O
₃ : 1 to 5%, F: 2 to 20%, MgO, B ₂
The total content of O ₃ and ZrO ₂ is 1 to 20%,
The total content of ₂ CO ₃ and Li ₂ CO ₃ is 2 to 20
%. The mold powder for continuous casting of steel according to claim 1, wherein 3. The ratio of the mass% of CaO to SiO ₂ Ca
O / SiO ₂ is 1.8 to 4.0 and in mass%, Ca
_{O: 10~70%, SiO 2:} 10~30%, Al 2 O
₃ : 5-20%, F: 5-30%, MgO, B
The total content of ₂ O ₃ and ZrO ₂ is 1 to 25%,
When the total content of a ₂ CO ₃ and Li ₂ CO ₃ is 2 to 2
The mold powder for continuous casting of steel according to claim 1, wherein the content is 5%. 4. The ratio of the mass% of CaO to SiO ₂ Ca
O / SiO ₂ is 5.5 to 7.0 and in mass%, Ca
_{O: 10~70%, SiO 2:} 5~15%, Al 2 O
₃ : 10 to 40%, F: 10 to 30%, Mg
The total content of O, B ₂ O ₃ and ZrO ₂ is 1 to 30
%, Continuous casting mold powder of the steel according to claim 1, wherein the total content of Na ₂ CO ₃ and Li ₂ CO ₃ is 2 to 30%.