JP3317258B2 - Mold powder for continuous casting of high Mn round section slabs - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-Mn round-section slab which is capable of obtaining a high-quality slab having few bubble defects generated immediately below the skin in continuous casting of a round-section slab. The present invention relates to a continuous casting mold powder and a continuous casting method using the same.

[0002]

2. Description of the Related Art In continuous casting of steel, mold powder is added to the surface of molten steel poured into a mold from an immersion nozzle. Usually, as the mold powder, a powder obtained by mixing a plurality of kinds of powders of oxides, carbon materials, or the like, or after mixing, is formed into granules.

[0003] The mold powder put into the mold plays the following roles. (1) Insulation of molten steel and prevention of oxidation, (2) Absorption of bubbles and inclusions in molten steel, (3) Ensuring lubricity between mold inner wall and solidified shell, (4) Clearance of gap between mold inner wall and solidified shell Adjustment of thermal conductivity.

[0004] In order to achieve such a role, the composition of the mold powder is examined according to the casting conditions, and characteristics such as melting / slagging speed, viscosity and solidification temperature during melting, and crystallization ratio after solidification are determined. Adjusted.

In the case of continuous casting of a slab having a round cross section, bulging (expansion due to static pressure of molten steel) is small because the solidified shell is annular, so that the solidified shell has a smaller shape than a slab having a rectangular cross section. Poor adhesion to mold wall. Therefore, when the molten powder flowing into the gap between the mold wall and the solidified shell is excessively crystallized, the glass quality in the powder film becomes insufficient, and the filling property between the mold wall and the solidified shell gap is lost. Many voids are created between the wall and the solidified shell. For the above reasons, the powder used for continuous casting of round section slabs must not be excessively crystallized when solidifying after melting and flowing into the mold-shell gap. If the crystallization is excessive, the mold
This is because many voids are generated between the shells, and the mold cooling ability at the void generating portion is significantly reduced locally, so that the solidification of the slab becomes uneven and surface defects such as vertical cracks occur.

Under these circumstances, conventionally, for example, CAMP I
SIJ VOL.8 (1995) -1013 and Japanese Patent 7-214263 discloses, as shown in JP-A-8-25008, the continuous casting of round cross-section billet, the CaO * and SiO ₂ Mold powder having a chemical composition that suppresses crystallization during solidification of molten powder by limiting the weight ratio CaO * / SiO ₂ (hereinafter, basicity) to 0.9 or less has been applied. Note that the above CaO * is the total Ca
O-converted weight.

[0007]

However, Mn in steel
When casting a steel with a concentration of 0.8 mass% or more (hereinafter referred to as high Mn steel), if mold powder with a basicity of 0.9 or less is applied, SiO ₂ in the molten powder oxidizes Mn in the steel.
The study of the present inventors has clarified that the following [Formula (1)] progresses, and MnO 2 is generated and enriched in the powder molten layer.

1 / 2SiO ₂ + Mn → MnO + 1 / 2Si (1) When the initial SiO ₂ concentration and the Mn concentration are high and the MnO concentration and the Si concentration are low, the reaction of the equation (1) is as follows. It proceeds until a certain equilibrium state is reached.

FIG. 1 shows the Mn concentration in steel ([% Mn]), the concentration of MnO enriched in the powder molten layer (% MnO), and the change in basicity that occurred before the powder before use became molten powder. Δ (CaO / Si
O ₂ )].

Here, the chemical composition of the powder melted layer (melted powder) was determined by collecting and analyzing a thin film in which the molten powder flowing into the gap between the mold and the solidified shell became a thin film and was discharged from the mold outlet.

In this investigation, C: 0.2 ma was used as the steel component.
The molten steel used was ss% and Mn: 1.4 mass%, and the powder used at that time was that of Comparative Example G in Table 1 described later.
From FIG. 1, the basicity increases (SiO ₂ decreases) and MnO is concentrated during casting of a high Mn steel, indicating that the reaction shown in the above equation (1) is proceeding.

Further, FIG. 2 shows the results of an investigation in a laboratory using a crucible to investigate the relationship between the SiO ₂ concentration in the powder before melting and the MnO concentration enriched in the molten powder layer after the melting of the powder. Show.

From FIG. 2, it was confirmed that when powder having a low SiO ₂ concentration was used, MnO was hardly enriched in the slag in which the powder was melted. This result also shows that
This supports that O ₂ generates MnO 2 through the reaction of the above-mentioned formula (1).

As described above, mold powder containing a large amount of SiO ₂ , that is, having a low basicity generates MnO. It is believed that the MnO thus generated reacts with the carbon compounded as a slagging inhibitor in the powder, and generates CO gas by the reaction of the following formula (2).

MnO (Molten Powder) + C (Powder Powder) → Mn (Molten Steel) + CO (Case) (2) Especially when the thickness of the powder molten layer is thin and the level of the molten metal in the mold changes greatly The unslagified powder containing carbon and the molten powder enriched in MnO react with the molten steel, and a CO gas is generated by the reaction of the above formula (2), which is taken into the solidified shell and easily becomes a cellular defect.

In a continuous casting machine, the molten steel in a mold is rotated by electromagnetic stirring (hereinafter, referred to as EMS).
It is widely practiced to increase the number of equiaxed crystals by generating crystal nuclei to reduce porosity (shrinkage cavities) and component segregation in the center of the slab. However, when a rotating flow is given by EMS in a round cross-section mold, the molten steel near the mold wall rises due to centrifugal force as schematically shown in FIG. 3, so that the powder molten layer at this portion becomes thinner. Increases the rate of occurrence of cellular defects. This corresponds to the transfer of Mn to the molten steel according to the above equation (2).

In the figure, the molten steel 14 injected from the immersion nozzle 12 into the mold 10 generates a rotating flow by EMS indicated by an arrow,
It can be seen that the surface of the molten steel near the mold wall is raised. As a result, the molten layer 18 of the powder formed below the powder layer 16 is thinned in the vicinity of the mold, and promotes the reaction of the formula (2) between the unslagged powder, the molten powder, and the molten steel, This results in increased cellular defects.

In fact, as shown in FIG. 4, the occurrence of the cellular defect is remarkable in a high Mn steel as a matter of course. FIG. 4 is a graph showing the Mn concentration in the molten steel and the tendency to generate cellular defects indicated by an index. It can be seen that the generation of the cellular defects is proportional to the Mn concentration in the molten steel.

SUMMARY OF THE INVENTION It is an object of the present invention to prevent a bubble defect that easily occurs when a high Mn steel is cast while rotating molten steel in a mold, particularly in a continuous casting machine having a round section mold. To provide a mold powder that can be used.

[0020]

Means for Solving the Problems In order to solve the above problems, in order to prevent bubble defects generated through the reactions of the above-mentioned equations (1) and (2), the reaction of the equation (1) It is easily presumed that it is effective to reduce the SiO ₂ concentration in the mold powder with the intention of suppressing the progress.

With respect to such reduction of the SiO ₂ concentration,
Conventionally, as shown in JP-A-57-184563, in the case of casting aluminum-killed steel, 7.0 weight SiO ₂ concentration mainly of Al ₂ O ₃ -CaO the purpose of suppressing oxidation and the like of Al in the molten steel %
The following reduced mold powder has been proposed. Further, Japanese Patent Application Laid-Open No. Sho 60-133396 discloses that Al ₂ O ₃ , CaO and
A mold powder based on MgO and containing no SiO ₂ has been proposed.

At first, the present inventors have taken thorough measures to suppress the reaction represented by the formula (1) and prevent bubble defects.
Since it was considered necessary to reduce the SiO ₂ concentration, a mold powder similar to the one disclosed in JP-A-57-184563 or JP-A-60-133956 was trial-produced and tried to be used for experiments. However, it is very difficult for these low SiO ₂ powders to adjust the characteristics such as the viscosity and solidification temperature of the molten slag or the crystallization rate after solidification so as to be suitable for continuous casting of round-section slabs. In the range of several tens of brands prototyped by the inventors, it was impossible to obtain powder that could be actually used for casting.

Here, the characteristics of powder suitable for continuous casting of a round section slab are as follows. The viscosity of molten slag at 1300 ° C. is as disclosed in Japanese Patent Application Laid-Open No. 8-25008.
0.3 to 0.7 Pas, solidification temperature is CAMP-ISIJ VoL.8 (199
5) As disclosed in -1013, for example, when the carbon concentration is 0.2
When casting 0.5 mass% of carbon steel from
About 0 to 1150 ° C., and the degree of crystallization is required not to be excessive as described above.

In view of the fact that it has been difficult to achieve a thorough reduction of the SiO ₂ concentration in this way, the present inventors have made various studies and found that even if the SiO ₂ concentration was not thoroughly reduced, the SiO ₂ concentration could not be reduced. It was thought that if the activity of No. ₂ could be reduced to about 1/2, the above-mentioned bubble-like defect could be reduced to a level having no practical problem.

For example, in FIG. 1, the Mn concentration in steel is 0.6
When casting about 2% steel, although about 2% MnO is enriched in the molten powder, as shown in FIG.
If the Mn concentration in the steel is about 0.6%, the occurrence rate of bubble defects is practically negligible.

From the viewpoint of suppressing the reaction from the left side to the right side in equation (1), a decrease in the Mn concentration in steel (strictly speaking, the Mn activity) is equivalent to a decrease in the SiO ₂ activity in the powder.
If the activity of SiO ₂ can be reduced to half, for example, Mn
There is a possibility that the amount of O generated can be suppressed to a level that does not cause any problem. Here, the activity is obtained by multiplying the molar concentration by the activity coefficient and is an index indicating the chemical reaction activity.

[0027] The present inventors have focused on the activity of SiO ₂ in the _{CaO-SiO 2 -Al 2 O 3} 3 -way system, which is a main component of mold powder, when the basicity exceeds 0.9, the basicity increases Against
It has been found that the SiO ₂ activity decreases rapidly. In addition, conventional round-section continuous casting powders, when the basicity exceeds 0.9, excessively crystallize during solidification and hinder casting.On the other hand, in the case of powders with increased Al ₂ O ₃ concentration, The present inventors have discovered that Al ₂ O ₃ as an oxide acts as an acidic oxide and suppresses crystallization, and thus completed the present invention.

Here, the present invention is as follows. (1) Mn: A mold powder for continuous casting of high-Mn round section slabs containing 0.8 mass% or more, where CaO * is the weight of total Ca in terms of CaO, and the weight ratio of CaO * to SiO ₂ , CaO * Mold powder characterized by having a chemical composition satisfying: / SiO ₂ = 0.95 to 1.40 Al ₂ O ₃ concentration Al ₂ O ₃ = 10 to ₃₀ mass% Na ₂ O concentration Na ₂ O ≦ 4 mass%

(2) Mn: 0.8 while performing electromagnetic stirring in the mold
The continuous casting of a high Mn round section slab is characterized in that, when continuously casting a high Mn round section slab containing at least mass%, the mold powder described in (1) above is poured into the molten steel surface in the mold. Method.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the chemical composition of powder in the present invention as described above will be described. In the powder according to the present invention, the main components CaO and SiO ₂ are CaO *
As long as the / SiO ₂ ratio (basicity) satisfies 0.95 to 1.40, the amount of each component is not particularly limited. In addition, CaO * is the value obtained by converting all the Ca content in the powder into CaO.

[0031] Conventional, Al ₂ basicity becomes excessive crystallization at the time of solidification and melting powder exceeds 0.9, although the continuous casting of round billet was found to be incompatible, amphoteric oxide Since O ₃ acts as an acidic oxide in this composition range, even if the basicity exceeds 0.9 if Al ₂ O ₃ is contained at 10 mass% or more, no adverse effect due to excessive crystallization occurs. Became clear. In the present invention, the lower limit is set to 0.95 because a practical effect is exhibited when the basicity is 0.95 or more.

On the other hand, if the basicity (CaO * / SiO ₂ ) exceeds 1.40, it becomes difficult to suppress crystallization even if the Al ₂ O ₃ concentration is increased beyond 30 mass%. Here, when the Al ₂ O ₃ concentration exceeds 30 mass%, the viscosity of the molten powder increases as shown in FIG.
Although the viscosity decreases as the added amount of F increases, an increase in the added amount of F causes adverse effects such as an increase in the erosion of the immersion nozzle in the mold. Therefore, the Al ₂ O ₃ concentration is limited to a maximum of 30 mass%. Preferably the concentration of Al ₂ O ₃ is 10-20%.

As shown in FIG. 6, when the basicity increases, the coagulation temperature increases. Therefore, in order to maintain an appropriate coagulation temperature, it is necessary to increase the amount of F added. On the other hand, an increase in the amount of added F causes adverse effects such as an increase in the melting damage of the immersion nozzle in the mold. In addition, F is compounded as fluorite or the like, and is a coagulation temperature and a viscosity lowering agent, and may be usually compounded at 3 to 10% by mass.

Under these circumstances, the upper limit of the basicity (CaO * / SiO ₂ ) was set to 1.40. Since Na ₂ O reacts with SiO ₂ and generates a liquid phase at a relatively low temperature of about 800 ° C., it becomes a binder and causes the formation of a sintered body called a slag rim 82 as shown in FIG. In the drawing, the unslag-free powder 80 forms a sintered body when Na ₂ O is contained. This sintered body is the slag rim 82. Since slag rims hinder operations such as powder supply and confirmation in a mold, it is desirable not to produce slag rims. From this viewpoint, the Na ₂ O concentration was limited to 4 mass% or less.

As shown in FIG. 8, the Na ₂ O concentration was 4 mass
% Or less, the growth of the slag rim is small and no operational problems occur. Since other alkali metal oxides such as K ₂ O and Li ₂ O also have the same action, when they are blended, it is desirable that they have a low concentration.

The powder according to the present invention may contain MgO for adjusting the coagulation temperature and viscosity, or as an impurity such as Al ₂ O ₃ raw material, and C for adjusting the slagging rate, if necessary. It is good, but there is no particular limitation on the inclusion thereof as long as it is contained in ordinary powders such as carbonates.

According to a preferred embodiment of the present invention, the powder according to the present invention generally has the following composition. _{CaO * / SiO 2: 0.95~1.40,} Al 2 O 3: 10~30mass%, Na 2 O: 0 ~ 4 mass%, F min: 3 ~ 10mass%, MgO: 0 ~ 5 mass%, C content: 2 ~ 7 mass%.

However, CaO * is a CaO converted value of the total Ca amount, and the F component is the F component of the raw material to be blended as fluorite or the like.
The C content takes into account only the added carbon such as coke powder and carbon black and the carbon content of the raw material blended as the carbonate. Next, the working effects of the present invention will be described more specifically with reference to examples.

[0039]

Examples Table 1 shows examples of the present invention and comparative examples.
The slab size was 225 mm in diameter, the casting speed was 2.0 m / min,
EMS was performed with a plane rotating flow.

[0040]

[Table 1]

Examples AF are examples of powder compositions according to the present invention. In the present invention, since the purpose is to reduce the activity of SiO ₂ and suppress the oxidation of Mn, which is an alloy component in steel, Fe ₂ O _{3 and the} like, which are oxides lower than SiO ₂ , are naturally used. However, it is not added, and it is preferable to suppress the concentration to a low concentration of 2 mass% or less.

When casting was performed using the powders of Examples A to F, the MnO concentration in the powder molten layer could be suppressed to 2 mass% or less, which was a problem-free low concentration. Furthermore, since the molten powder flows into the space between the mold and the solidified shell and then solidifies properly when solidified, the heat flux fluctuation in the mold during casting is small. In addition, the growth of slag rim is small, and there are no operational and quality problems.

On the other hand, Comparative Example G, which is an example of a powder that has been widely used in the continuous casting of cast slabs having a round cross section, is CaO *
Since the ratio of / SiO ₂ is small and the concentration of Al ₂ O ₃ is low, the activity of SiO ₂ is high. As a result, the MnO concentration in the powder melted layer was high and caused a quality problem.

In Comparative Example H, although the CaO * / SiO ₂ content was as high as 1.00, the concentration of MnO in the molten powder was low, but the concentration of Al ₂ O ₃ acting as an acidic oxide was as low as 5.5%, so that the molten powder was not. However, since excessive crystallization occurs during solidification after flowing into the gap between the mold and the solidified shell, there has been a problem that heat flux fluctuations in the mold during casting become large. If the heat flux variation in the mold is large, local cooling unevenness occurs, resulting in quality and operation troubles such as cracking defects on the slab surface and breakout due to slab cracking.
Further, in Comparative Example H, since the concentration of Na ₂ O was high, the slag rim grew greatly and hindered the operation.

Comparative Example I is an example of the invention disclosed in JP-A-57-184563 having CaO-Al ₂ O ₃ as a main composition. This powder has a low SiO ₂ concentration, so the MnO concentration in the powder melt layer is low, but it has a very high basicity, and the crystallization suppression during solidification was insufficient even with an increase in Al ₂ O ₃ concentration. . Therefore, the heat flux fluctuation in the mold during casting is large,
Application to actual operations was difficult. Furthermore, since the concentration of F added for adjusting the physical properties is as high as 10 mass%, there is an operational problem that the molten steel injection nozzle immersed in the mold has a large melt damage.

In Comparative Example I, although the Na ₂ O concentration was as high as 5 mass%, the growth of slag rim hardly occurred. this is,
This is probably due to the low concentration of SiO ₂ that forms a low melting point liquid phase by reacting with Na ₂ O.

[0047]

As described above, the powder according to the present invention satisfies various functions required for continuous casting of a slab having a round cross section and, at the same time, has a low MnO concentration in a powder molten layer at the time of high Mn steel casting. Good slab surface properties can be obtained without generation of bubble defects caused by MnO.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the Mn concentration in molten steel, the MnO concentration in a molten powder layer, and a change in basicity.

FIG. 2 SiO ₂ concentration of powder before melting and Mn in molten powder
4 is a graph showing a relationship with O concentration.

FIG. 3 shows that when centrifugal force is applied,
FIG. 9 is a schematic explanatory view showing a state where the transfer of Mn is promoted.

FIG. 4 is a graph showing the relationship between the Mn concentration in molten steel and the tendency to generate cellular defects.

FIG. 5 is a graph showing the relationship between Al ₂ O ₃ concentration and viscosity at 1300 ° C.

FIG. 6 is a graph showing the relationship between basicity and solidification temperature.

FIG. 7 is a schematic explanatory view of a slag rim forming mechanism.

FIG. 8 is a graph showing the relationship between Na ₂ O concentration and slag rim growth rate.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-284749 (JP, A) JP-A-50-77209 (JP, A) JP-A-4-224063 (JP, A) JP-A-4-27 319056 (JP, A) JP-A-63-188459 (JP, A) JP-A-7-214263 (JP, A) JP-A 8-25008 (JP, A) JP-A-57-184563 (JP, A) JP-A-60-133956 (JP, A) JP-A-11-320058 (JP, A) JP-A-5-185195 (JP, A) JP-A-9-85404 (JP, A) (58) (Int.Cl. ⁷ , DB name) B22D 11/108 B22D 11/00 B22D 11/115

Claims (2)

(57) [Claims] 1. A mold powder for continuous casting of a high-Mn round section cast slab containing Mn: 0.8 mass% or more, wherein CaO * is a total Ca
When the CaO-converted weight, CaO * the weight ratio of _{SiO 2, CaO * / SiO 2} = 0.95~1.40 Al 2 O 3 concentration _{Al 2 O 3 = 10~30mass% Na} 2 O concentration Na ₂ O ≦ 4mass% of A mold powder having a satisfactory chemical composition. 2. Mn: 0.8 ma while performing electromagnetic stirring in the mold.
When continuously casting high Mn round section slabs containing ss% or more,
A method for continuously casting a high-Mn round-section slab, wherein the mold powder according to claim 1 is poured into a surface of molten steel in a mold.