JPH105953A - Front powder for steel continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent breakout at an early casting phase and to improve slab surface quality by specifying the property of front powder in a prescribed range when a medium carbon steel is subjected to continuous casting. SOLUTION: When the medium carbon steel of 0.08-0.18wt.% carbon content is subjected to continuous casting, the front powder, which has <=1000 deg.C solidification temp., >=930 deg.C melting temp., 3.5-7.0g/cm<2> .min melting rate, is used. When a solidification temp. is higher than 1000 deg.C, formation of uneven solidified film is progressed to lose effect. When a melting temp. is lower than 930 deg.C, a melting rate is turned to excessive or white smoke is generated, slag foaming is suddenly increased to lose its effect. In the case of <=1.5 poise viscosity, partial void is generated between a mold and slab, resulting in uneven cooling. Crack is generated in the case of >=40 poise, breakout is generated in the case of <=3.5g/cm<2> .min melting rate, operation trouble is caused in the case of >=7.0g/ cm<2> .min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for forming a slag film which melts at an appropriate speed to form a slag layer to cover a molten steel surface and to form a lubricating action in an early stage of continuous casting of steel. This is related to front powder used for stable operation and producing excellent slabs.Especially for continuous casting of medium carbon steel, it has the greatest effect on preventing breakout at the early stage of casting and cracking on the slab surface. I do.

[0002]

2. Description of the Related Art In recent years, continuous casting of steel has been accelerated in order to save energy and resources, to reduce costs by improving yields and unit consumption, and to cool and reheat slabs produced by high-speed continuous casting. The direct rolling method of feeding directly into the rolling process has been actively adopted. In order to perform this direct rolling, it is necessary to minimize defects in the cast slab, and research has been conducted.However, breakouts and cracks occur in the early stage of casting for medium carbon steel and peritectic composition steel. It is easy to do and remains a big problem.

In order to prevent the occurrence of the cracks, it is necessary to make the cooling of the slab uniform, and for that purpose, the solidification temperature of the powder is raised to flow between the mold and the slab. By increasing the heat transfer resistance of the slag film itself, it is effective to increase the heat transfer resistance from the slab to the mold and slowly cool the slab. However, even if such a powder is used, in the early stage of casting,
Due to a decrease in the temperature of the molten steel or a shortage of the molten slag layer, the lubricity of the slag is insufficient or uneven inflow occurs, and constrained breakouts and cracks occur more frequently than in a normal operation. Thus, improvement of the main body powder alone has not solved these problems.

In order to solve these problems, a front powder used at the start of casting has been proposed. That is, as a means for securing a stable operation in the early stage of casting, a method of using a heat generating powder in the early stage of casting has been adopted.
The powder used in the early stage of the casting is disclosed in, for example, JP-B-57-7211 and JP-B-6-73728, each of which is capable of rapidly exhibiting the original function of the powder, and those having an exothermic reaction. As the description indicates that these powders need to fulfill the role of powder, these front powders do not specially alter the molten slag properties of general powders (body powders), except for heat generation. Recognize. In the invention of the above-mentioned official gazette having the function of generating heat as described above, at present, it is not possible to sufficiently prevent restraint breakout or cracking in the early stage of casting of medium carbon steel.

[0005]

SUMMARY OF THE INVENTION The present invention relates to a casting method having a specific function required in the early stage of casting in order to prevent breakout in the early stage of casting and cracks in the surface of the slab which could not be solved by the prior art. The present invention provides a front powder to be used at an early stage. Particularly, in continuous casting of medium carbon steel, it exerts a maximum effect on prevention of breakout at an early stage of casting and crack on a slab surface.

[0006]

Means for Solving the Problems The present inventors have conducted research on preventing breakout and cracking of the surface of a slab in the early stage of continuous casting of medium carbon steel. A front powder was found and found to be able to prevent breakout at the beginning of casting and solve cracks on the slab surface by using it at the beginning of casting.

That is, the means 1 has a carbon content of 0.08
When casting medium carbon steel of ~ 0.18 wt% by continuous casting,
Solidification temperature is 1000 ° C or less, melting temperature is 930 ° C or more,
Viscosity; 1.5 to 4.0 poise, melting rate;
The front powder is 7.0 g / cm ² · min.

Means 2 is that the component composition is expressed as CaO / SiO ₂ = 0.60-1.00 (% by weight)
Ratio), Al ₂ O ₃ ; 3 to 7% by weight, Na ₂ O;
0.6 to 13% by weight, Li ₂ O; 0.5% by weight or more,
F; 6 to 12 wt%, Fe ₂ O _3; 7-14 wt% unavoidable free carbon is 1.0 wt% or less, within its chemical composition, 7-14 wt% iron oxide as an oxidizing agent Mix,
Melting rate of Ca-Si alloy as exothermic agent is 3.5-7 g
/ Cm ² · min.

Further, the means 3 is the same as the means 2 except that Li ₂ O
Is 5% by weight or less.

[0010]

DETAILED DESCRIPTION OF THE INVENTION The carbon content is between 0.08 and 0.18.
In the continuous casting of medium-weight carbon steel by weight, due to the low molten steel temperature and the low casting speed in the early stage of casting, the optimum powder (body powder) in the steady state of the slab is overgrown or uneven in the solidified film thickness. It was found that solidification, insufficient inflow, growth of slag bear, etc. occurred, which caused breakout and cracks on the slab surface.

The general method of forming a solidified film to achieve uniform cooling by slow cooling is rather ineffective at the beginning of casting, and uniform cooling by a mold through a lubricating film is very effective. The solidification temperature of the powder needs to be 1000 ° C. or less.

When the solidification temperature is higher than 1000 ° C., the formation of a non-uniform solidified film proceeds and the effect is lost. In addition, since this front powder has a much lower solidification temperature than the main body powder used later, after fulfilling the initial purpose, it flows out of the mold and is smoothly replaced by the main body powder with a high solidification temperature, One of the characteristics is that no adverse effect is seen by using powders of different types continuously.

Further, the melting temperature is set to 930 ° C. or higher because if the melting temperature is lower than this, the melting rate becomes too high, or the generation of white smoke due to the volatilization of the flux components and the rapid rise of slag foaming during melting. In addition to being practical, the intended effect cannot be obtained.

Since the viscosity is low at the beginning of casting, the viscosity is 1.5 po
If the temperature is less than "ise", molten slag flows out, and a partial gap is formed between the mold and the slab, resulting in uneven cooling. 4.0pois
Above e, smooth replacement with the main body powder is hindered, and cracks occur.

In order to supply a required amount of front powder having such characteristics in the early stage of casting and to obtain an optimum melting state, the melting rate is set to 3.5 to 7.0 g / cm ² · min.
Need to be Melting rate is 3.5 g / cm ² · mi
Below n, breakout occurs due to lack of molten slag required for lubrication. Melting rate is 7.0 g / cm ² · m
In the case of in or more, the slag surface once in a semi-molten state is cooled and solidified into a lump or a plate to cause an operation trouble.

The present invention further enhances the effect by:
The present invention proposes a front powder capable of obtaining a stable melting rate and a stable melting state. That is, when the solidification temperature is 1000
The viscosity is 1.5 to 4.0 poise at a temperature of not more than 0 ° C. In order to suppress generation of white smoke and foaming due to volatilization of a flux component, 0.6 to 1.0% by weight of CaO / SiO ₂ is used.
It becomes possible by setting the ratio and Al ₂ O ₃ to 3 to 7% by weight. When the content of CaO / SiO ₂ is 0.6% by weight or less, Al ₂
When O ₃ is 7% by weight or more, Na is added to decrease the viscosity.
Flux components such as ₂ O and F become excessive. CaO / S
When the content of iO ₂ is 1.0% by weight or more and the content of Al ₂ O ₃ is 3% by weight or less, it is difficult to lower the solidification temperature and maintain the viscosity within the range.

0.6 to 1.0% by weight of CaO / SiO ₂
In terms of the ratio, when the Al ₂ O ₃ content is ₃ to 7% by weight, the solidification temperature is 1 due to the flux components of Na ₂ O, F and Li ₂ O.
000 ° C or lower, melting temperature 900 ° C or higher, viscosity 1.5
Adjustment to ~ 4.0 poise is possible.

When the content of Na ₂ O is 7% by weight or less, the solidification temperature or viscosity cannot be sufficiently lowered, and when the content of Na ₂ O exceeds 13% by weight, white smoke and foaming increase rapidly. F is 5
If it is less than 12% by weight, the viscosity will be too high, and if it is more than 12% by weight, the viscosity will be low, and white smoke and foaming will increase.

As a component for suppressing this white smoke and foaming, Li
₂ with O is very effective, as shown in FIGS. 1 and 2, to prevent the solidification temperature of the powder is lowered,
This has the effect of suppressing the occurrence of surface cracks and breakout of the slab. However, if the content of Li ₂ O is 0.5% by weight or less, the effect is insufficient, and if the content exceeds 5% by weight, the effect is in a saturated state, and it is not necessary to add further from the viewpoint of economy.

In order to stably melt the front powder of these components at the melting rate of 3.5 to 7.0 g / cm ² · min, it is effective to use heat generation. This exotherm reduces unavoidable free carbon, resulting in Ca-S
It is optimal to use the oxidation-reduction reaction of the combination of the i-alloy and iron oxide with little influence on the components and characteristics.
If the content of iron oxide is 7% by weight or less, the melting rate is low. If it exceeds 14% by weight, the melting rate is too high or white smoke is generated.

The amount of the Ca-Si alloy is determined so that the melting rate is 3.5 to 7.0 g / cm ² · min. The amount varies depending on the components and the raw material system, but is about 12 to 20% by weight. Further, if the unavoidable free carbon content exceeds 1.0% by weight, the melting rate becomes low, and 3.5 g / cm ² · min cannot be secured. By imparting heat in this manner, a stable melting rate can be obtained even when a raw material that undergoes an endothermic reaction in the melting process such as a carbonate is used.

Further, the front powder of the present invention is made of 100 me
The raw material of the powder whose sh-passing weight% is 95% or more is used. The method of use and the amount of use are only used in the early stage of casting, and are added when the molten steel surface covers the discharge holes of the immersion nozzle, and the amount of use is 7 to 15 mm / cm ² in terms of molten slag per molten steel upper area. Add an amount corresponding to the thickness.
Then, in part, the body powder is added when the melt reaches the upper surface.

The melting rate is set to 15
1.0 g / cm on 10 kg of molten iron maintained at 50 ° C.
^It was determined from the time until the amount of the front powder of No. ² was charged and melted. The solidification temperature was 1300 by the rotating cylinder method.
The viscosity (η) was measured while lowering the temperature (T) at a rate of 1 ° C./min from 1 ° C., and the temperature was determined as the temperature at which the linear relationship between 1 / T and log η changed due to the sudden increase in apparent viscosity due to crystal precipitation. The melting temperature of the sample formed into a 5 mmφ × 5 mmh cylindrical shape was raised from 700 ° C. at a rate of 10 ° C./min.
It was the temperature at which a hemisphere of mmh was formed.

[0024]

The present invention will be described more specifically below with reference to examples and comparative examples of the front powder according to the present invention. Table 1 shows the results of use of the product of the present invention, the comparative product, and the actual machine. Casting conditions are: mold size 220-280 × 900-
1800 mm, the cast steel grade has a carbon content of 0.08-0.
18% medium carbon steel. The evaluation of the prevention of breakout is as follows: Including the charge in which slab burn-in was found even if it did not lead to the breakout by predictive detection, the rate of occurrence in the charge ratio was 0.2% or less. △, 2%
Those that exceeded were evaluated as ×. Regarding the evaluation of slab surface cracks, the care rate for vertical cracks with a length of 100 mm or more,
A sample having a slab length ratio of 1.0% or less was evaluated as ○, a sample having a slab length ratio of 3.0% or less was evaluated as △, and a sample exceeding 3.0% was evaluated as ×.

[0025]

[Table 1]

A, B and C in Table 1 are according to the present invention.
All showed good results. Comparative product D was low in melting temperature and viscosity, and contained no Li ₂ O and contained more Na ₂ O and F than in the range of the present invention. Therefore, white smoke was generated and the slab surface quality was poor. Comparative product E has good melting characteristics as a main body powder because the solidification temperature is lower than the range of the present invention and CaO / SiO ₂ is higher than the range of the present invention, but the non-uniform solidified film in the early stage of casting And, because of poor lubricity, many breakouts (including predictive detection) and no improvement in slab surface quality.

[0027]

According to the front powder of the present invention, it is possible to simultaneously prevent breakout in the early stage of casting of medium carbon steel and to improve the surface quality of the slab, which have been considered difficult in the past. Is to bring.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amount of Li ₂ O added and the solidification temperature of powder.

FIG. 2 is a graph showing the relationship between the amount of Li ₂ added and the rate of occurrence of surface cracks and breakouts of a slab.

 ──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Hideyuki Misumi 1 Nishinosu, Oita, Oita, Nippon Steel Corporation Oita Works (72) Inventor Hidehisa Taniguchi 114-5 Ushigami Nakatsu-shi, Oita

Claims (3)

[Claims] 1. A carbon content of 0.08 to 0.18% by weight
Solidification temperature is 1 when casting medium carbon steel by continuous casting
000 ° C or lower, melting temperature 930 ° C or higher, viscosity: 1.5
~ 4.0 poise, melting rate; 3.5-7.0 g / c
m ² · min, a front powder for continuous casting of steel. 2. The composition of the oxide is CaO /
In the range of SiO ₂ = 0.60 to 1.00 (weight% ratio),
Al ₂ O _3; 3~7 wt%, Na ₂ O; 0.6~13 wt%, Li ₂ O; 0.5% by weight or more, F; 6 to 12 wt%, Fe ₂ O _3; 7~14 % By weight, unavoidable free carbon is 1.0% by weight or less, and in its component composition, 7 to 14% by weight of iron oxide is mixed as an oxidizing agent, and C is used as a heating agent.
The melting rate of the a-Si alloy is 3.5 to 7 g / cm ² · mi.
2. The front powder for continuous casting of steel according to claim 1, wherein n is added. 3. The method according to claim 2, wherein the content of Li ₂ O is 5.
Front powder for continuous casting of steel up to 0% by weight.