JP2008200721A - Mold flux for continuous casting of steel, and continuous casting method using the mold flux - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mold flux for continuous casting of steel capable of realizing the consistent high-speed casting by consistently promoting solidification in a mold. <P>SOLUTION: In the mold flux for continuous casting of steel, the wetting angle θ(°) at 1,140°C of a molten flux with respect to a surface material of a mold is a function of the crystallization temperature Tcs(°C) of the flux, and expressed by inequalities : 0.12(Tcs-800)≤θ≤70. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a mold flux for continuous casting of steel and a continuous casting method using the mold flux.

  FIG. 5 is an explanatory view of a partial cross section in a mold of a vertical continuous casting machine for steel, showing a state in which mold flux (also referred to as a molten metal (surface) coating agent or mold powder) is added to molten steel in the mold. ing. Above the molten steel 1 is a molten flux 3 and further above that is an unmelted flux 5. And in the boundary surface with the casting_mold | template 7, there exists the solid-phase flux film | membrane 9 and the fusion | melting flux film | membrane 11 in an order from the near to the casting_mold | template 7, and the solidified shell 13 is formed in the inner side further.

  Mold flux mainly consists of (1) heat insulation and oxidation prevention on the surface of the molten steel in the mold, (2) absorption of non-metallic inclusions floating in the molten steel and cleaning of the molten steel, and (3) between the mold and the initial solidified shell. (4) It is added for the purpose of controlling the heat removal from the solidified shell and making it uniform.

In recent years, in order to improve productivity, the casting speed tends to be higher, and therefore, the residence time of molten steel in the mold is shortened, so that the thickness of the solidified shell of the steel at the mold outlet is also becoming thinner. Therefore, it is required to promote solidification in the mold by controlling the heat removal from the solidified shell by the mold flux.
However, there have been many proposals for methods related to stabilization of lubrication in the mold, but methods for promoting in-mold solidification include components that tend to vitrify after the solidification temperature of the mold flux decreases or the solidification of the molten mold flux. There were few methods (refer to patent document 1).
JP 2006-247712 A

In the conventional example, as the influence of the mold flux on the solidification of the steel, it is necessary to consider a plurality of factors such as the crystallization temperature of the flux, the vitrification degree (crystallization degree), and the type of crystal. As described above, the method of considering a plurality of factors does not always provide the expected result with an actual machine. For this reason, even when the flux design is designed with a low crystallization temperature during high speed casting to enhance the heat removal capability, the amount of heat removal from the mold does not increase when actually used, and solidification within the mold is sufficiently accelerated. In some cases, it was not possible.
On the other hand, there is also a problem that the amount of heat removal is excessively higher than expected, which promotes nonuniform solidification.

  The present invention has been made to solve the above-described conventional problems, and provides a mold flux for continuous casting of steel that can stably promote solidification in a mold and stabilize high-speed casting. With the goal.

The inventors have determined that the wetting angle θ of the molten mold flux with respect to the mold surface material (hereinafter sometimes simply referred to as “wetting angle θ” or “wetting angle θ of the mold flux”) and the solidification characteristics of the steel via the mold flux. As a result, it was found that the solidification rate of steel in the mold is greatly influenced by the wetting angle θ in addition to the crystallization temperature Tcs of the flux.
Based on the above knowledge, the inventors have cast an ultra-low carbon steel slab at high speed, and are keenly aware of the relationship between the occurrence of breakout (BO) in the relationship between the mold flux wetting angle θ and the crystallization temperature Tcs. Investigated and found that the presence or absence of breakout (BO) can be clearly arranged in the relationship between the wetting angle θ and the crystallization temperature Tcs, and the physical properties of the mold flux to make the initial solidification of steel fast and uniform and stable. It discovered and came to this invention.

(1) The mold flux for continuous casting of steel according to the present invention has the following equation in which the wetting angle θ (°) at 1140 ° C. of the melt flux with respect to the mold surface material is a function of the crystallization temperature Tcs (° C.) of the flux. It is characterized by being given.
0.12 (Tcs−800) ≦ θ ≦ 70

  Here, the wetting angle θ means that the molten flux held at 1430 ° C. for 60 minutes is solidified on a copper plate and 0.5 g made into 100 mesh under powder is compressed into a 10φ × 3 mm tablet with a compression force of 1 ton. 1 is set on a Co85% Ni15% plated SUS304 substrate as shown in FIG. 1, while introducing Ar gas in a horizontal tubular furnace at a heating rate of 5-10 ° C./min at 1140 ° C. during heating. The molten flux is filmed and defined as its contact angle θ.

(2) The continuous casting method according to the present invention is characterized by using the mold flux for continuous casting described in (1) above.

  According to the present invention, slab cooling can be made uniform and solidification in the mold can be stably promoted to stabilize high-speed continuous casting.

  As described above, the inventors diligently studied the relationship between the wetting angle θ of the molten mold flux with respect to the mold surface material and the solidification characteristics of the steel through the mold flux. It was discovered that the mold flux was greatly affected by the wetting angle θ of the mold flux and the crystallization temperature Tfcs of the mold flux.

FIG. 2 is a graph schematically showing the relationship between the wetting angle θ and the solidification characteristics of steel. The horizontal axis indicates the wetting angle θ, and the vertical axis indicates the heat transfer resistance (upper stage) and the shell nonuniformity in order from the upper stage. (Middle stage) and shell thickness (lower stage) are shown.
Hereinafter, the wetting angle θ of the melted mold flux with respect to the mold surface material and the solidification characteristics of the steel through the mold flux will be described with reference to FIG.
2, R is the overall thermal resistance between the solidified shell and the mold, λf is the thermal conductivity of the flux, Rint is the interface thermal resistance between the flux and the mold, and df is the total thickness of the flux.

When the melted mold flux is easily wetted with the mold (the wetting angle is small), the contact area between the melted mold flux and the mold is increased (interfacial thermal resistance Rint is decreased), and the heat transfer to the mold immediately after contact is increased. Promoted. For this reason, solidification of the melted mold flux proceeds rapidly, and the flux thickness df between the mold and the solidified shell grows thick. As a result, the heat transfer resistance increases (see the upper diagram).
Therefore, in the molten steel that solidifies thereafter, the solidified shell is more likely to grow more uniformly (see the middle figure).
However, since the heat transfer resistance is large, the shell growth rate is suppressed and the shell thickness does not increase (see the lower figure).
On the other hand, the same result is obtained when the flux is too wet and the flux crystallization temperature Tcs is low. Also in this case, although the interfacial thermal resistance Rint is small, the flux thickness df is difficult to increase. For this reason, the heat transfer resistance is further reduced and the cooling rate of the solidified steel is increased. As a result, the solidified shell tends to grow unevenly, and the shell thickness does not grow thick.

In such a case, if the flux crystallization temperature Tcs is high, the heat transfer resistance is further increased and the homogenization effect is promoted. On the other hand, the shell growth rate of the solidified steel is excessively suppressed and breakout (BO ) Is increased.
Therefore, for example, when using a flux with a high crystallization temperature Tcs to prevent cracking during high-speed casting of steel that is prone to surface cracking, such as medium carbon steel, the wetting angle is used to suppress such a shell thinning phenomenon. It is necessary to suppress the solidification of the flux using a large flux. That is, the minimum wetting angle for preventing breakout (BO) increases as the crystallization temperature Tcs increases.

On the other hand, when wetting is poor (wetting angle is large), the opposite phenomenon occurs when wetting is good. That is, since the contact area between the molten flux and the mold decreases (interfacial thermal resistance Rint increases), heat transfer to the mold immediately after contact is moderately suppressed, and solidification of the molten flux is also delayed. For this reason, the total flux thickness df between the mold and the solidified shell is reduced compared to the case where wetting is good. As a result, the overall heat transfer resistance of the whole (between the solidified shell and the mold) decreases (see the upper diagram).
By reducing the overall heat transfer resistance, solidification of the steel is promoted and the shell thickness is increased (see the lower figure).
Even if wetting is a little worse, there is little impact on the non-uniformity of the shell, but when wetting gets worse, the non-uniformity of the solidification of the steel is promoted, helping the non-uniformity of the contact state of the flux to the mold, The shell thickness is reduced (see the middle figure). As a result, breakout (BO) is likely to occur.
Therefore, there is an upper limit on the wetting angle in order to ensure the solidification uniformity of the steel.
In the present invention, the optimum value of the wetting angle has an upper limit of 70 ° for this reason.

The amount of heat removal: H, the solidified shell surface temperature of steel: Ts, the mold surface temperature: Tc, the overall thermal resistance between the solidified shell and the mold: R, the thermal conductivity of the flux: λf, the interfacial heat between the flux and the mold When the relationship between the resistance: Rint and the total flux thickness: df is expressed by a simplified formula, the following formula is obtained.
H = (Ts−Tc) / R = (Ts−Tc) / (Rint + df / λf)

  As described above, it has been found that the wetting angle θ is greatly influenced by the solidification of the steel other than the crystallization temperature Tcs of the flux.

Based on the above knowledge, the inventors of the present invention used an ultra-low carbon steel (ULC) (C: 0.0010 to 0.0030, Si <0.20, Mn = 0) in a vertical bending type steel continuous casting machine. 0.1 to 0.5, P: 0.005 to 0.030, S: 0.0001 to 0.015, Al: 0.01 to 0.04% by mass) Slab (size 220 to 235 mm × 1000 to 1800 mm) Was cast at a casting speed of 2.0 to 2.6 m / min, and the occurrence rate of breakout was investigated.
Table 1 summarizes the breakout (BO) generation rate (number of BO generation per charge) for each mold flux used.

  Further, FIG. 3 shows the result of Table 1 as a graph showing the occurrence of breakout (BO) in the relationship between the wetting angle θ (°) and the crystallization temperature Tcs (° C.). In FIG. 3, the vertical axis indicates the wetting angle θ (°), and the horizontal axis indicates the crystallization temperature Tcs (° C.). In FIG. 3, a white circle indicates a case where no breakout (BO) occurs, and a black circle indicates a case where a breakout (BO) occurs.

From the graph shown in FIG. 3, it can be seen that the presence or absence of breakout (BO) can be clearly arranged in the relationship between the wetting angle θ of the mold flux and the crystallization temperature Tcs.
In order to prevent the occurrence of breakout (BO), the relationship between the mold flux wetting angle θ (°) and the crystallization temperature Tcs (° C.) may be set so as to be the white circle region in FIG. .
That is, the mold flux wetting angle θ (°) and the crystallization temperature Tcs (° C.) are the physical properties of the mold flux that can quickly solidify and stabilize the steel to prevent the occurrence of breakout (BO). May be set so as to be in a range surrounded by a broken line in FIG. This relationship is expressed by the following equation (1).
0.12 (Tcs−800) ≦ θ ≦ 70 (1)

FIG. 4 shows the region in the graph of FIG. 3 divided into six regions A to F, and shows the state of the mold flux and the solidified shell at the time of casting when a mold flux having physical property conditions corresponding to each region is used. It is the figure shown typically. Below, based on FIG. 4, the state of the mold flux and the solidified shell in each area | region of FIG. 4 is demonstrated.
As can be seen from the above description, when the mold flux crystallization temperature Tcs increases, the solidification rate of the molten steel decreases. Conversely, when the mold flux crystallization temperature Tcs decreases, the solidification rate of the molten steel increases. .
Further, when the wetting angle θ is increased (wetting property is deteriorated), the tendency to non-uniform solidification and the tendency to suppress cooling. On the other hand, when the wetting angle θ is small (wetting property is improved), the tendency to uniform solidification and the tendency to promote cooling are obtained.

(1) Region A The physical properties of the region A are that the crystallization temperature Tcs is high and the wetting angle θ is small. In this region, since the crystallization temperature Tcs is high, the mold flux is quickly solidified, and the flux thickness df is increased. Moreover, since the wetting angle θ is small, the tendency of uniform solidification is strong for both flux and steel.
However, since the flux thickness df is large, the solidification of the molten steel is slow and the solidified shell thickness ds is thin, which may cause a breakout (BO).

(2) B region The B region has the same crystallization temperature Tcs as the A region, but has a larger wetting angle θ. By increasing the wetting angle θ, heat transfer to the mold immediately after contact is suppressed, and solidification of the molten flux is delayed, so that the flux thickness df is reduced and the heat transfer resistance is reduced. Therefore, the molten steel tends to be non-uniformly solidified, but the solidified shell thickness ds becomes larger than that in the region A, and breakout (BO) is avoided.

(3) C region The C region has the same crystallization temperature Tcs as the A and B regions, but the wetting angle θ is larger than that of the B region. By further increasing the wetting angle θ, the contact state of the flux with the mold becomes more uneven, and the thermal resistance between the flux and the mold (interfacial thermal resistance Rint) increases. As a result, the flux thickness df becomes thinner than that in the B region, but the overall heat transfer resistance R increases. Therefore, the solidified shell thickness ds is thin and non-uniform, and a risk of breakout (BO) occurs.

(4) D region The D region has the same wetting angle θ as in the A region, but has a lower crystallization temperature Tcs. Since the crystallization temperature Tcs is low and the wetting angle θ is small, the steel is rapidly cooled although the flux thickness df grows uniformly and thinly. As a result, the thermal shrinkage of the solidified shell is increased and nonuniform solidification is promoted. In this case, the average thickness of the solidified shell is larger than that in the A region, but the minimum thickness is not much different from that in the A region, and there is a risk of occurrence of breakout (BO).

(5) E region The E region has the same crystallization temperature Tcs as in the D region, but has a larger wetting angle θ. Since the crystallization temperature Tcs is low, the flux thickness df is thin, but by increasing the wetting angle θ, the interfacial thermal resistance Rint is increased, the cooling rate of the molten steel is less than in the D region, and the non-uniform solidification of the molten steel is caused. Alleviated. For this reason, the solidified shell is uniformly thickened, and the occurrence of breakout (BO) is avoided.

(6) F region The F region has the same crystallization temperature Tcs as the D and E regions, but the wetting angle θ is larger than that of the E region. By further increasing the wetting angle θ, the contact state of the flux with the mold becomes more uneven, and the thermal resistance between the flux and the mold increases. As a result, the flux thickness df becomes thinner than that in the E region but becomes non-uniform, so the solidified shell thickness ds also becomes thin, and the solidification of the molten steel becomes non-uniform, which causes a risk of breakout (BO). .

  As described above, it can be qualitatively explained that the risk of occurrence of breakout (BO) is avoided in the B and E regions in the A to F regions, which is a mold that satisfies the relationship of the above-described equation (1). The use of flux provides a theoretical support for the ability to avoid the occurrence of breakout (BO).

  Note that the wetting angle defined by θ shown in FIG. 1 generally has temperature dependence. Therefore, in this invention, it was set as the value in 5-10 degree-C / min at 1140 degreeC. This is because at 1140 ° C. or higher, θ becomes too small and measurement becomes difficult.

  The substrate material is ideally a coating material (Cr, Ni, FeNi, CoNi, etc.) that is generally used on the surface of the mold copper plate, but the CoNi plating that has almost no flux reaction at the above-mentioned temperature is optimal. is there.

As the wetting angle measurement flux, in consideration of the actual phenomenon, measurement flux can be suppressed by using a solidified flux once melted. In general, once the melted material is melted again (melted twice), there is a phenomenon that the crystallization temperature Tcs is lowered. Therefore, the flux crystallization temperature Tcs (viscosity is measured after the first melting). Melting from the lower temperature side than the rapidly rising temperature). Therefore, the contact angle can be measured even at 1140 ° C. lower than the crystallization temperature Tcs measured after the first melting. The crystallization temperature Tcs defined in the present invention is the crystallization temperature Tcs measured after the second melting, and generally refers to the temperature at which the viscosity rapidly rises when the temperature drops (5 to 10 ° C./min). .
In the present invention, continuous measurement was performed at a temperature drop rate of 5 ° C./min with a rotary viscometer, and the temperature at which the viscosity increased rapidly was defined as the crystallization temperature Tcs.

  Therefore, the wetting angle θ of the present invention is obtained by solidifying the molten mold flux held at 1430 ° C. for 60 minutes on a copper plate and compressing 0.5 g of 100 mesh under powder into a 10φ × 3 mm tablet with a compression force of 1 ton. The shaped product is set on a SUS304 substrate (plate thickness 0.8-1.0 mm cold-rolled plate) plated with Co 85% Ni 15% (thickness 0.2 mm), and Ar gas (purity 99.999) in a horizontal tubular furnace. The shape of the molten flux at 1140 ° C. during heating was introduced at a rate of temperature increase of 5-10 ° C./min while introducing a flow rate of 1 Nl / min) and defined as the contact angle θ.

  The measurement temperature of 1140 ° C. is one standard, but this value is not particularly necessary. If the crystallization temperature Tcs of the flux becomes too high or extremely low, the measurement becomes impossible. Therefore, in order to avoid this, the reference temperature (1140 ° C.) may be raised and lowered to a temperature at which measurement is possible. In that case, needless to say, the optimum wetting angle also decreases depending on the measurement temperature.

  Although the wetting angle θ is affected by the substrate material, the mold surface temperature during actual use in an actual machine is as low as 250 to 400 ° C. Therefore, when the mold surface material is a metal or alloy, the mold surface material is wet. The effect on the corner is negligible. Therefore, even if the wetting angle measured using CoNi plating as a substrate is used as a reference, it can be ignored that the degree of the effect of the wetting angle on the breakout (BO) is influenced by the mold surface material actually used.

  Moreover, since the thickness of the coating material on the surface of the mold meniscus portion is as thin as 0.2 μm to 0.5 mm, the ratio of the coating material to the total thickness of the mold copper plate (20 to 50 mm) is small, and its own thermal resistance can be ignored. Therefore, the optimum wetting angle discovered in the present invention does not change due to the difference in the thickness of the surface material.

The mold flux of the present invention is suitable for preventing the occurrence of BO during high-speed casting of steel that tends to solidify unevenly, and can be applied to casting of general ultra-low carbon steel and medium carbon steel.
In addition, the component composition range of a general very low carbon steel is shown below.
C: 0.0010 to 0.0030, Si <0.20, Mn: 0.1 to 0.5, P: 0.005 to 0.030, S: 0.0001 to 0.015, Al: 0. 01-0.04 mass%

It is sectional drawing which shows the wetting angle measuring method of mold flux. It is a conceptual diagram explaining the wettability of mold flux and the solidification characteristic of steel. It is a figure which shows the relationship between the wetting angle (theta) which influences a breakout (BO), and the flux crystallization temperature Tcs. The region in the graph of FIG. 3 is divided into six regions A to F, and schematically shows the state of the mold flux and the solidified shell at the time of casting when a mold flux having physical property conditions corresponding to each region is used. It is a figure. It is principal part sectional drawing of a continuous casting mold.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Molten steel 3 Molten flux 5 Unmelted flux 7 Mold 9 Solid phase flux film 11 Molten flux film 13 Solidified shell

Claims (2)

A mold flux for continuous casting of steel, characterized in that the wetting angle θ (°) at 1140 ° C. of the molten flux with respect to the mold surface material is given by the following equation as a function of the crystallization temperature Tcs (° C.) of the flux.
0.12 (Tcs−800) ≦ θ ≦ 70 A continuous casting method for steel using the mold flux for continuous casting according to claim 1.

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