JPH05228596A - Method for evaluating powder for continuous casting of low-carbon steel - Google Patents

Abstract

PURPOSE:To provide the index to elucidate the correlative relation with the heat extraction strength in a casting mold by determining the temp. at the boundary between the crystal phase and liquid phase of powder. CONSTITUTION:The selection of the powder at the time of continuously casting the low-carbon having 0.01 to 0.08wt.% carbon content affects the generation of the longitudinal crack on one surface of an ingot. The heat by a heating element 12 is regulated and supplied to a furnace 11 having a temp. gradient and the powder crystal phase 15 and molten powder 16 are made to coexist affinely in a crucible 14. The boundary surface temp. of the powder crystal phase 14 and the molten powder 16 is then determined. The boundary temp. indicates the temp. at the boundary of the solid phase and molten part of the powder film existing at the time of actual casting. The temp. of the plate in the casting mold is gentler as the above-mentioned temp. is higher.

Description

Detailed Description of the Invention

[0001]

The present invention has a carbon content of 0.01 to
The present invention relates to an evaluation method of 0.08% by weight (hereinafter abbreviated as "%") so-called low-carbon steel powder for continuous casting, which is a continuous method for obtaining a slab with no vertical cracks on the surface of the slab. Powder for casting (hereinafter abbreviated as powder)
For designing.

[0002]

2. Description of the Related Art Low carbon steel having a carbon content of 0.01 to 0.08% by weight is used as a plate material for automobiles, and therefore has a high demand for the surface quality of a slab, and its continuous casting is usually extremely difficult. Since high productivity is required, it is not as sensitive as so-called medium carbon steel, but surface slab cracks and breakouts often occur.

When such vertical surface cracks or breakouts occur, heat removal and lubrication in the mold are inappropriate. That is, it is considered that when the heat removal strength in the mold is too high, the thermal stress generated in the solidification shell is large, and when the heat removal heat in the mold is too low, the solidification shell is thin and its strength is low. Further, when the inflow of powder is obstructed and so-called powder breakage occurs, a large amount of stress is generated between the solidified shell and the mold, and the solidified shell is broken.

In order to avoid the above situation, the method usually used in continuous casting is the use of proper powder. In selecting the powder, the viscosity of the powder is usually used as an index of the lubricity of the powder, and the freezing temperature of the powder is used as an index of the cooling strength.

The solidification temperature of this powder is obtained by determining the viscosity by rotating a cylinder in a crucible held at a constant temperature, and in the figure plotting the viscosity against the measured temperature, the viscosity rapidly increases as the temperature decreases. It is said that the temperature will rise. It is thought that this rapid change in viscosity is due to the fact that the powder crystallizes as the temperature decreases and the apparent viscosity increases, and when this solidification temperature is high, the crystalline phase (fixed phase) in the powder film It is said that because of the large thickness, the thermal resistance between the mold and the solidified shell becomes large, so that slow cooling can be realized.

[0006]

The indicators of the heat removal in the mold and the solidification temperature according to the prior art are, for example, a high solidification temperature powder for moderate cooling used for medium carbon steel and a strong solidification temperature for low carbon steel. It was useful for comparing powders with different heat removal strengths, such as the powders of. However, with the recent tightening of slab surface flaws, it is necessary to minimize the spatial and temporal variations in heat removal strength, or to control heat removal strength with high accuracy. Low and insufficient. The reason for this is that the solidification temperature, which is measured by the change in viscosity, cannot reproduce the morphology of crystal crystallization and the supercooling ability of the crystal, and therefore it does not necessarily affect the heat removal strength in the mold. This is because the thickness is not shown.

Furthermore, most of the low carbon materials are so-called low carbon aluminum killed steels which are deoxidized by aluminum after converter refining, so that aluminum in the molten steel reduces SiO ₂ content in the powder at the molten metal surface. However, the powder components are largely changed by the inclusion of the floating Al ₂ O ₃ inclusions in the powder. For this reason, powders with similar ingredients and with the same solidification temperature,
In the case of casting under the same casting conditions, it is often the case that the same heat removal strength in the mold cannot always be obtained.

For this reason, after all, by adjusting the heat removal strength in the mold by designing and developing the powder, the chemical composition of the powder as the base is changed little by little, and it is actually applied. Was the main approach, and was extremely inefficient. Further, from the viewpoint of avoiding unexpected accidents at the time of testing, powder change tests involving major component changes are often avoided, and it is often impossible to make drastic changes to powder.

On the other hand, according to the present invention, by indexing the thickness of the stationary phase in the powder film formed between the solidified shell and the mold, which is considered to be obtained during casting, and the magnitude of heat removal strength, It is made to provide a method for easily and surely controlling the heat removal strength in a mold for the purpose of reducing surface defects and breakout of a continuously cast slab.

[0010]

[Means for Solving the Problems] The present inventors have studied the relationship between the characteristic value of powder for low carbon steel, the crystal structure in the powder film, the heat removal strength in the mold, and the vertical crack of the surface of the slab. As a result of stacking, it was found that there is a good correlation between the heat removal strength in the mold and the interface temperature between the crystals in the powder and the molten powder, and the present invention was completed based on this finding.

According to the present invention, when controlling the heat removal strength in the mold by changing the powder for low carbon steel, the powder crystal phase and the molten powder are made to coexist in a furnace having a temperature gradient, and the powder crystal phase and the melting powder are melted. This is a powder design / evaluation method characterized in that the powder interface temperature is obtained and the powder is evaluated with good visibility as an evaluation index.

[0012]

The present invention will be described below with reference to the drawings.

FIG. 1 is a view showing the state of powder in the mold (1). Powder (2) supplied from above the bath surface
Is melted by the heat supplied from the molten steel (5) on the molten metal surface (4) to form a molten pool (3). The melted powder flows between the mold (1) and the solidification shell (6) to form a powder film (8) having a solid phase on the mold side and a liquid phase on the solidification shell side. In addition, a part of the granule powder is sintered by heat to form a slug bear (7).

FIG. 2 shows a powder film (8) taken from between the mold and the solidified shell. As described above, the powder film is composed of a solid phase (fixed phase) and a liquid phase (molten phase) during casting, but since it was sampled while lowering the molten metal surface, it was all in the solid phase. However, when the tissue is observed in the portion (10) below the meniscus vicinity portion (9), the boundary can be clearly known.

The heat conduction between the fixed phase and the molten phase is worse in the fixed phase, and since the thickness of the fixed phase is usually thicker than that of the molten phase, the thickness of the fixed phase is dominant in heat removal in the mold. . or,
As can be seen from the fact that vertical cracks occur in the initial stage of solidification, it is important to control the thickness of the pinned layer in this portion in order to control the heat removal strength near the meniscus.

FIG. 3 (a) shows a furnace (11) having a temperature gradient, in which heat is supplied from a heating element (12) and the powder crystal phase (15) and the molten powder (1) are simulated in the furnace.
6) is allowed to coexist in the crucible (14) and the interface temperature between the powder crystal phase and the molten powder is determined. Further, FIG. 3B shows a temperature distribution in the furnace which has been measured in advance. The output of the furnace is adjusted so that a predetermined temperature gradient is obtained, and a crucible in which molten powder is cast is inserted into the furnace and held for a certain period of time. The holding time varies depending on the output of the furnace, but 10 to 30 minutes is suitable for reproducing the conditions almost the same as those of the stationary phase of the powder film.

After holding for a certain period of time, the sample is pulled out by the elevating device (13) or the furnace is lifted to rapidly cool the sample. FIG. 4 shows the crystalline phase (15) and the molten powder (16) of the sample in the crucible held in the furnace having the temperature gradient of FIG.
It is an enlarged cross-sectional structure of the boundary part of the crystal grain boundary and is divided into a crystal part (17) in which the crystal grain boundaries are linear and coarse and a part (18) in which the crystal grows like a dendrite. It is the same tissue that is observed within.

In continuous casting of ordinary steel, C, SiO
A powder containing ₂ , Al ₂ O ₃ , CaO, Na ₂ O, F, etc. as a main component is used. In such powder, the crystal part is mainly composed of CaO, SiO ₂ , and F, which is called caspodyne, and further, Al is present between the crystal grains.
There is a phase containing ₂ O ₃ , Na ₂ O, and SiO ₂ as main components.

The crystals in the powder film are those crystallized from the molten powder, and the phase filling the spaces between the crystals is due to the redistribution of the solute that accompanies the crystallization of the powder. The difference in crystal grain size is due to the difference between the cooling rate during crystallization and the holding time at high temperature. That is, the part where the crystal grain is large and the grain boundary is linear is
This is due to the coarsening that occurs after being crystallized and then exposed to high temperature conditions, while the portion that grew in the dendrite shape was due to the high cooling rate. It is a part that is rapidly cooled and solidified by the operation of the equipment or the pulling up of the furnace body.

That is, the interface (19) between the two regions is the interface between the crystal and the liquid phase, and the interface temperature between the crystal and the liquid phase can be known from the position of the interface in the sample and the temperature distribution in the furnace. . Since this interface temperature represents the temperature of the interface between the fixed phase and the molten portion of the powder film that actually exists during casting, the higher this temperature, the slower the heat removal in the mold.
By designing the powder while evaluating the powder based on this index, it becomes possible to efficiently design the powder capable of producing a slab with significantly less vertical cracking on the slab surface.

[0021]

EXAMPLES Examples according to the present invention will be described below. Table 1 shows the components of the low carbon steels used in Examples and Conventional Examples,
Table 2 shows the components of the main powders used in Examples and Conventional Examples.
Shown in. Using the above low carbon steel and powder components,
Casting was performed in an actual continuous casting machine under the following conditions. Mold vibration condition: stroke 6.8 mm, oscillation cycle = 55 × Vc + 50, drawing speed: 0.9-2.
0 m / min, slab size: thickness 240 mm, width 1000
~ 2200mm, Tundish molten steel heating degree 10
30 ° C.

Test casting was performed under the above conditions to select the powder. The powder used had a viscosity at 1300 ° C of 2.0-2.5 poise and a solidification temperature of 103.
It was 0-1170 degreeC. FIG. 5 shows the relationship between the interface temperature and the vertical cracking occurrence index at that time. When the interface temperature is lower than 1100 ° C (△), cracks are more frequently generated, but this is because the fixing layer is thin in the powder film and the heat removal was high, resulting in fine vertical cracks and horizontal cracks. Further, when the interface temperature is higher than 1200 ° C., the heat removal strength was too low and the solidified shell strength was insufficient, causing vertical cracking.

From this figure, a powder having an interface temperature of about 1100 to 1200 ° C. is suitable for suppressing surface cracking. In this embodiment, the powder is changed to (◎) in the figure in consideration of the occurrence of pinholes and blowholes on the surface, but the generation of vertical cracks can be clearly suppressed compared to when the old powder (○) is used. ing. Moreover, the occurrence of breakout could be suppressed.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

EFFECTS OF THE INVENTION By using the method for evaluating a mold powder according to the present invention, heat removal in a mold is made appropriate,
It becomes possible to efficiently design a powder capable of producing a slab with significantly less vertical cracking on the slab surface.

Further, as shown in FIG. 5, when the relationship between the surface vertical crack and the interface temperature is clarified, the performance of the powder of the component system which has never been used up to now according to the method of the present invention is offline. You can know. Further, if the reactivity between the powder and the molten steel can be known in advance, more accurate prediction can be performed. For this reason, it becomes possible to prevent stable casting and to avoid a dangerous actual molten metal test in which vertical cracks frequently occur.

[Brief description of drawings]

Fig. 1 is a schematic diagram showing the state of the molten metal surface, granule powder, solidified shell and powder film in the continuous casting mold during casting, Fig. 2 is an overall view of the powder film, and Fig. 3 (a) is the sticking in the powder film. A schematic diagram of a furnace used for indexing the layer thickness, FIG. 3 (b) is a diagram of temperature distribution in the furnace, FIG. 4 is a structural diagram near the solid-liquid interface, and FIG. 5 is an interface temperature. It is a characteristic view showing a relation with a vertical crack occurrence index.

Claims (1)

[Claims] Claim: What is claimed is: 1. When designing a powder for continuous casting used for low carbon steel having a carbon content of 0.01 to 0.08% by weight, heat removal in a mold by determining the temperature of the powder crystal phase and liquid phase interface. A method for evaluating powder for continuous casting of low-carbon steel, which is characterized by using it as an index of strength.