JP5397213B2 - Continuous casting method - Google Patents

Description

The present invention relates to a continuous casting method for manufacturing a slab by supplying molten steel to a mold.

Conventionally, when electromagnetic steel containing 1.0% by mass or more of Si is cast with a continuous casting machine, the surface temperature of the slab is lowered due to uneven cooling in the width direction of the slab generated in the secondary cooling zone. As a result, surface cracks occur in the slab. This crack is caused by overcooling of the slab and is usually dealt with by reducing the amount of water injected in the secondary cooling zone. However, in electromagnetic steel with a high Si content, when the amount of water injection in the secondary cooling zone is reduced, internal cracks in the slab due to bulging between rolls that cause product surface defects occur.
For this reason, in the adjustment of the water injection amount in the secondary cooling zone, it is difficult to achieve both internal cracks due to insufficient cooling and surface cracks due to supercooling. As a method for making the surface temperature of the slab uniform, it is already known that the powder (also referred to as mold powder or mold flux) is peeled off from the surface of the slab directly under the mold in addition to the adjustment of the water injection amount described above. ing.

For example, Patent Document 1 describes a continuous casting method in which a mold flux and an oxide film attached to a slab surface are removed between a mold and the first support roll. This removal is performed by causing a water jet to collide with the slab surface at a collision pressure of 10 N / cm ² or more using a high-pressure spray nozzle.
Further, in Patent Document 2, calculation is performed using the surface temperature of the solidified shell on the exit side of the mold, the solidification temperature of the mold flux introduced into the molten steel in the mold, and the content of the chemical component of the mold flux. A method is described in which continuous casting is performed by adjusting the casting speed so that the β value satisfies 1.5 or less or 2.5 or more.

JP 2003-275852 A JP 2005-324214 A

However, the conventional method still has the following problems to be solved.
In the method of Patent Document 1, when water injection of 10 N / cm ² or more is performed with a high-pressure spray nozzle, particularly in steels with high Si content (for example, non-directional steels), surface cracks in the slab due to supercooling Is likely to occur and is difficult to apply.
Moreover, when using the method of patent document 2, since the flow rate of spray water is very large just under the mold, it is difficult to continuously measure the surface temperature of the slab at the lower end of the mold. When the surface temperature of the slab is adjusted by the casting speed, in the case of electromagnetic steel having a high Si content, internal cracks occur due to bulging of the solidified shell, which causes surface defects of the product.
Further, the methods of Patent Documents 1 and 2 are based on the idea that cooling is hindered by powder adhesion, and cooling in the high-speed casting (Vc ≧ 2.0 m / min) region by peeling of the powder. In order to improve the capacity, it has been difficult to solve the surface cracks of the slab by uneven cooling in the width direction of the slab.

The present invention was made in view of such circumstances, by improving the peelability of the powder itself, stabilizing the cooling in the width direction of the slab, suppressing the surface cracks of the slab caused by overcooling, An object of the present invention is to provide a continuous casting method capable of producing a high quality slab.

In the continuous casting method according to the present invention that meets the above object, molten steel containing 1.0% by mass or more of Si is supplied to a mold, and the consumption of powder supplied into the mold is 0.2 kg / m ² or more and 0.0. In the continuous casting method of 6 kg / m ² or less,
After the powder is heated and melted, the solidification temperature, which is the temperature at which crystals begin to crystallize in the process of lowering the temperature, is set to 1050 ° C. or more and 1200 ° C. or less, and when the molten powder is rapidly cooled and solidified, the resulting glass is annealed The crystallization temperature, which is the temperature at which crystals begin to precipitate, is set to 500 ° C. or more and 600 ° C. or less .

Continuous casting method according to the present invention, since the consumption of powder supplied into the mold to 0.2 kg / m ² or more 0.6 kg / m ² or less, ensure proper deposition thickness of the powder slab And overcooling of the slab can be prevented.
Further, since the solidification temperature of the powder is 1050 ° C. or more and 1200 ° C. or less and the crystallization temperature is 500 ° C. or more and 600 ° C. or less, the liquid phase and crystal phase constituting the slag film formed between the mold and the solidification shell And each thickness of a glass phase can be adjusted. Thereby, the slag film can be prevented from falling off between the mold and the solidified shell, so that the seizure between the mold and the solidified shell can be suppressed.
Therefore, when continuously casting molten steel containing 1.0% by mass or more of Si (for example, electromagnetic steel such as non-oriented electrical steel), surface cracks can be suppressed while preventing breakout due to seizure during continuous casting. High quality slabs can be manufactured stably.

It is explanatory drawing which shows the relationship between the cooling capacity index | exponent of a slab by the presence or absence of powder adhesion, and the distance from a casting_mold | template. (A), (B) is explanatory drawing which showed typically the slag film formed between molten steel and a copper plate, when the solidification temperature and crystallization temperature of powder were each changed. It is explanatory drawing which showed the influence which solidification temperature and crystallization temperature have on continuous casting.

Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention.
In the continuous casting method according to an embodiment of the present invention, molten steel containing 1.0% by mass or more of Si is supplied to a mold, and the amount of powder consumed in the mold is 0.2 kg / m ² or more and 0.0. a method for the 6 kg / ^{m 2} or less, the solidification temperature of the powder and 1050 ° C. or higher 1200 ° C. or less, a process for the 500 ° C. or higher 600 ° C. or less crystallization temperature.
Hereinafter, the background to the present invention will be described.

A continuous casting machine (hereinafter also referred to as a continuous casting machine) has a mold and a secondary cooling zone disposed from directly under the mold in the casting direction, and the secondary cooling zone is for cooling the slab. A plurality of cooling nozzles are arranged. When continuously casting electromagnetic steel containing 1.0% by mass or more of Si (upper limit is, for example, 4% by mass) using this continuous casting machine, this electromagnetic steel is easily embrittled at a high temperature. Internal cracks occur at the solidification interface due to the strain that occurs when the slab swells due to the static pressure of molten steel between the rolls arranged in the band. In order to prevent this cracking, it is necessary to suppress bulging between rolls by increasing the amount of water from the cooling nozzle disposed in the secondary cooling zone and increasing the thickness of the solidified shell.

However, when the amount of cooling water is increased, the cooling performance is improved, and supercooling and surface cracking occur in the slab. This tendency becomes prominent particularly when Si is 2.8% by mass or more (further 3.5% by mass or more). For this reason, it is difficult to adjust both the internal cracks due to insufficient cooling and the surface cracks due to supercooling when adjusting the amount of water in the secondary cooling zone.
Therefore, the present inventors pay attention to the releasability of the powder (mold powder) from the surface of the slab as a factor given to cooling in the secondary cooling zone, and the powder necessary for achieving uniform cooling of the slab. The physical properties were examined.

Generally, the powder supplied into a mold having a contact surface side of molten steel made of a copper plate (made of copper or copper alloy) is an artificial slag made of SiO ₂ , CaO, F, Na ₂ O, etc., and is dispersed on the molten steel. It melts with the heat of molten steel. The powder thus melted flows between the mold of the continuous casting machine and the solidified shell (solidified steel), and serves as a lubricant for preventing seizure between the mold and the solidified shell.
There have been many reports that when this powder is cooled in the secondary cooling zone while adhering to the surface of the slab, the cooling capacity of the slab decreases. This is because the powder having a lower thermal conductivity than the slab becomes thermal resistance and inhibits heat removal from the slab itself.

However, as a result of investigations by the present inventors, it has been found that when powder adheres to the surface of the slab, the cooling ability may be improved. Hereinafter, the results of investigation by the present inventors on the influence of the presence or absence of powder adhesion on the cooling ability of the slab by a laboratory test will be described.
First, in the laboratory test, after the steel material in which the thermocouple was embedded was heated to 1200 ° C. or higher, this was cooled with a cooling nozzle, and the influence of the cooling rate of the steel material depending on the presence or absence of powder adhesion was investigated. Subsequently, using the results of this laboratory test and the heat transfer analysis model, the effect of the presence or absence of powder adhesion on the cooling capacity of the slab was investigated. In addition, the heat transfer analysis model used used the general method shown by iron and steel, a 60th volume (1974), p. 1023, for example.

Here, the structure of the continuous casting machine which analyzed, casting conditions, and cooling conditions are shown below.
・ Pitch of rolls from directly under the mold of continuous casting machine to 1.2m in casting direction: 200mm
・ Distance from directly under mold to bent back part: 16m
Casting conditions: casting speed 1.3 m / min, casting width (slab width) 1300 mm, casting thickness (slab thickness) 250 mm
- cooling conditions: from the mold immediately below, in the range of up to 2.0m in the casting direction, the water density of the cooling water sprayed onto the cast piece from the cooling nozzle, ² per surface area of the slab 1 m, 450 l / min (hereinafter, L / m ² / min)) and constant.

The analysis result is shown in FIG.
The horizontal axis in FIG. 1 indicates the distance (m) in the casting direction from directly below the mold. In FIG. 1, the distance from the casting direction to 2.0 m (slab surface temperature of 900 ° C.) immediately below the mold (the value of the horizontal axis is 0.0 m: the surface temperature of the slab is approximately 600 ° C.). The range is illustrated.
In addition, the vertical axis in FIG. 1 indicates that when the slab is cooled on the premise that powder adheres, the heat transfer coefficient of the slab where the distance in the casting direction is equivalent to 1.2 m is 1, and no powder adheres. The values obtained by indexing (cooling capacity index) the respective heat transfer coefficients of the slab in the case of (solid line in FIG. 1) and attached (dotted line in FIG. 1) are shown.

First, the knowledge obtained from FIG. 1 is shown below.
-Cooling ability decreased with the progress of casting (increase in the horizontal axis) regardless of the presence or absence of powder adhesion.
-Although there is a magnitude relationship of cooling ability depending on the presence or absence of powder adhesion, the magnitude relationship may differ depending on the position of the horizontal axis (position where the surface temperature of the slab is different).
In the vicinity of the mold (for example, at a position of 0.2 m), the cooling ability index was about 30% larger when no powder was adhered than when powder was adhered.
-When the position of 1.2 m was exceeded in the casting direction from directly under the mold (for example, the position of 1.4 m), the cooling capacity index was smaller than 20% when the powder was not adhered, compared with the powder adhered.

Thus, it was found that the magnitude relationship of the cooling capacity changes at the intersection A depending on the presence or absence of powder adhesion even at the same distance from directly under the mold. That is, when powder adheres to the surface of the slab, the surface temperature of the slab decreases and the temperature deviation in the width direction increases, and one or both of internal cracks and surface cracks occur.
In the actual casting test, when the powder consumption was less than 0.2 kg / m ² , the temperature deviation in the width direction of the slab increased, and the surface crack of the slab occurred. This is presumed that when the amount of powder consumption decreases, the thickness of the powder adhering to the surface of the slab immediately below the mold becomes thin, and the pressure from the spray water makes it easy to peel from the surface of the slab. In addition, even if the spray water density is the same, if the powder consumption exceeds 0.6 kg / m ² , the effect of suppressing the temperature deviation in the width direction of the slab will be saturated, so it is necessary to increase the powder consumption further. The nature is low.

From the above, the consumption of powder of (supply amount) 0.2 kg / ^{m 2} or more 0.6 kg / ^{m 2} or less (preferably, a lower limit, 0.25 kg / ^{m 2,} even 0.3 kg / ^{m 2} By setting the upper limit to 0.55 kg / m ² , and further 0.5 kg / m ² ), it is possible to secure an appropriate thickness of attachment to the slab and to stabilize the cooling of the slab. Become.
The adjustment of the consumption amount of the powder was realized mainly by changing the amounts of CaO, SiO ₂ , F and Na ₂ O in the powder and controlling the viscosity. This viscosity can be adjusted to an arbitrary value while fixing a solidification temperature and a crystallization temperature described later.

The powder consumption (kg / m ² ) is defined as follows.
{Powder consumption (kg / m ² )}
= {Amount of powder put into meniscus during casting time (kg)}
/ {Casting speed (m / min) × {slab width (m) + slab thickness (m)} × 2 × casting time (min)}
Here, the casting time means, for example, a time for casting one charged molten steel of about 150 to 350 tons or a time for casting a plurality of charged molten steels.

As shown above, the powder consumption can be controlled to make the cooling of the slab uniform in the width direction. However, because seizure occurs frequently between the mold and the solidified shell, stable casting is possible. The problem remained from the point of view.
Therefore, powder physical properties were adjusted for the purpose of suppressing seizure between the mold and the solidified shell. In the present embodiment, the solidification temperature and the crystallization temperature are used as indices for evaluating the powder. This solidification temperature is the temperature at which the crystal begins to crystallize in the process of lowering the temperature after the powder is heated and melted, and the crystallization temperature is when the glass produced when the molten slag is rapidly cooled and solidified is annealed. The temperature at which crystals begin to precipitate.

2A and 2B schematically show the state of the molten slag flowing between the mold (copper plate) and the solidified shell of the continuous casting machine, that is, the state of the slag film. 2 (A) and 2 (B), the solidified shell formed on the slag film side of the molten steel is not shown.
As shown in FIG. 2, generally, the slag film is divided into three phases in the order of a glass phase, a crystal phase, and a liquid phase (molten state) from the mold side. Here, the solidification temperature (BP) is the boundary between the liquid phase and the crystal phase constituting the slag film, and the crystallization temperature (Tc) is the boundary between the crystal phase and the glass phase constituting the slag film, respectively. Represents.

Therefore, in the present embodiment, the casting stability of the high-Si steel is achieved by controlling the thickness of the three phases of the liquid phase, the crystal phase, and the glass phase. In addition, adjustment of the crystallization temperature and the solidification temperature is performed by changing the blending of CaO, SiO ₂ , F, Na ₂ O, Li ₂ O, and Al ₂ O _{3 in} the powder. It was possible to arbitrarily control the viscosity, the crystallization temperature, and the solidification temperature.
First, the crystallization temperature (Tc) of the powder was adjusted by mainly increasing or decreasing the amounts of Na ₂ O and F in the powder. This crystallization temperature has a smaller range that can be controlled by changing the components than the solidification temperature. As is clear from FIG. 2, when the solidification temperature is constant, the glass phase decreases and the crystal phase thickness increases when the crystallization temperature is lowered.

As a result of the actual machine test, when the crystallization temperature is lower than 500 ° C., the thickness of the crystal phase increases, the thickness of the liquid phase with respect to the crystal phase decreases relatively, and the frictional resistance between the slag film and the slab is reduced. For example, when the mold is oscillated, the slag film is dropped from between the mold and the solidified shell. As a result, since the mold and the solidified shell are in contact with each other, the heat removal behavior in the mold becomes unstable, causing operational troubles such as a restraint breakout.
On the other hand, when the crystallization temperature exceeds 600 ° C., the thickness of the crystal phase becomes thin and the glass phase becomes thick. Then, overcooling occurred in the slab.
From the above, the crystallization temperature was regulated in the range of 500 ° C. or more and 600 ° C. or less to adjust the solidification temperature.

Regarding the physical properties of the powder, if the solidification temperature exceeds 1200 ° C., a constraining breakout occurs. This is because the crystal phase becomes thicker and the liquid phase becomes thinner, so the lubricity between the mold and the solidified shell deteriorates, the slag film falls off, and the mold and the solidified shell come into contact with each other. It is estimated that.
On the other hand, when the solidification temperature was lower than 1050 ° C., the molten metal surface level became remarkable and the surface cracks of the slab were frequently generated. This is because when the solidification temperature is lowered, the liquid phase is thickened and the crystal phase is thinned, so that the radiation heat transfer between the solidified shell and the mold is increased, the cooling capacity in the mold is improved, and the initial solidification is performed. This is due to destabilization and non-uniform growth of the solidified shell.

The uneven solidification that occurs in the early stage of solidification is promoted by the secondary cooling zone, and causes fluctuations in the molten metal surface due to bulging between rolls. Moreover, it is estimated that the surface crack of a slab is because if the liquid phase becomes too thick, the amount of powder attached to the slab surface increases and the peelability is hindered.
For operational troubles such as fluctuations in the molten metal surface, breakout, and surface cracks in the slab, the casting speed can be reduced to less than 0.6 m / min. This is a problem because it becomes an obstructing factor.

Therefore, by applying the powder having the above physical properties, stable casting can be performed at a casting speed of 0.6 m / min or more without affecting productivity. The upper limit of the casting speed is not specified, but at present, the casting speed of the molten steel may be set at 3.0 m / min.
From the above results, from the results of tests conducted using an actual continuous casting machine, the solidification temperature is 1050 ° C. or higher and 1200 ° C. or lower (preferably, lower limit is 1080 ° C., further 1100 ° C., upper limit is 1170 ° C., 1150 ° C.) and a crystallization temperature of 500 ° C. or more and 600 ° C. or less (preferably, the lower limit is 520 ° C., and the upper limit is 580 ° C.).

When casting electromagnetic steel containing 1.0% by mass or more of Si with a continuous casting machine, the powder adhering to the solidified shell is effectively peeled off, resulting in non-uniform cooling in the width direction generated in the secondary cooling zone. The surface crack of the slab caused by the occurrence occurs.
However, by using the powder having the above-mentioned physical properties, the surface temperature of the slab at the correction point (position where correction from the curved portion to the horizontal portion) is 600 to 900 ° C. Both crack prevention can be achieved. This is because if the surface temperature of the slab falls below 600 ° C., surface cracks occur in the slab, whereas if it exceeds 900 ° C., the internal crack of the slab deteriorates to a level that affects the product. .

Next, examples carried out for confirming the effects of the present invention will be described.
Here, when test casting was carried out on an electromagnetic steel containing 3.0% by mass or more of Si with a continuous casting machine, powders with variously changed solidification temperatures and crystallization temperatures were supplied into the mold and evaluated.
The continuous casting machine used is a vertical bending type slab continuous casting machine in which a number of divided rolls are arranged on the downstream side of the mold in the casting direction. Within this mold, various powders, its consumption is supplied such that 0.2 kg / ^{m 2} or more 0.6 kg / ^{m 2} within the range, the casting speed of the slab 0.8~1.5m / Min. The cross-sectional size of the cast slab is as follows: thickness: 252 mm, width: 1000-1500 mm.
Table 1 and FIG. 3 show the types of powder used in the test casting and the casting results.

In Table 1, the powders of Examples 1 to 6 were prepared by adjusting the solidification temperature within the range of 1050 ° C. to 1200 ° C. and the crystallization temperature within the range of 500 ° C. to 600 ° C. ( (Within the hatched area in FIG. 3). On the other hand, each powder of the conventional example and Comparative Examples 1 to 5 has a solidification temperature or a crystallization temperature outside the above-described range (outside the shaded area in FIG. 3). The conventional powder is an existing powder.

Table 1 shows evaluation results of seizure, hot-water surface fluctuation, and supercooling. For each of these evaluations, a case where continuous casting can be continuously performed at a casting speed of 0.6 m / min or more is indicated by “◯”, while seizure, molten metal surface fluctuation, or surface cracking occurs. A case where it was judged that it was difficult to continuously carry out a casting speed of 6 m / min or more and it was necessary to reduce the casting speed to less than 0.6 m / min was indicated by “x”.
The determination shown in Table 1 indicates that the case where continuous casting can be continuously performed at a casting speed of 0.6 m / min or less is marked with “◯”, while the casting speed is reduced to a casting speed of less than 0.6 m / min. The cases where it is necessary to do so are indicated by “x” marks.

From Table 1 and FIG. 3, in the conventional example using the existing powder, slab cracking due to overcooling of the slab occurred.
Further, in Comparative Examples 1, 2, 4, and 5, the occurrence of surface cracks in the slab was suppressed, but seizure occurred in the mold, and a breakout precursor phenomenon (leaching of molten steel to the slab surface occurred. ) In some cases. In Comparative Example 3, no seizure occurred in the mold, but the molten metal surface fluctuation was large and the operability was poor.
For this reason, in the conventional example and Comparative Examples 1-5, it was necessary to reduce a casting speed to less than 0.6 m / min.

On the other hand, in Examples 1 to 6, the occurrence of supercooling of the slab can be suppressed, no operational troubles due to fluctuations in the heat removal from the mold are observed, and continuous casting can be continued at a casting speed of 0.6 m / min or more. Met.
In addition, although test casting was performed in the range of 1000 to 1500 mm in the width of the slab, continuous casting could be carried out without changing the casting speed. In particular, the improvement effect when casting a slab having a wide width of 1200 to 1500 mm, in which seizure and hot-water surface fluctuation easily occur, was remarkable.
From the above, it has been confirmed that by using the continuous casting method of the present invention, operational troubles such as seizure, molten metal surface fluctuation, and supercooling can be avoided and the productivity of the slab can be improved.

As described above, the present invention has been described with reference to the embodiment. However, the present invention is not limited to the configuration described in the above embodiment, and the matters described in the scope of claims. Other embodiments and modifications conceivable within the scope are also included. For example, the case where the continuous casting method of the present invention is configured by combining some or all of the above-described embodiments and modifications is also included in the scope of the right of the present invention.

Claims (1)

Supplying a molten steel containing Si more than 1.0 mass% in a mold in a continuous casting process for the consumption of powder supplied to 0.2 kg / ^{m 2} or more 0.6 kg / ^{m 2} or less in the template,
After the powder is heated and melted, the solidification temperature, which is the temperature at which crystals begin to crystallize in the process of lowering the temperature, is set to 1050 ° C. or more and 1200 ° C. or less, and when the molten powder is rapidly cooled and solidified, the resulting glass is annealed The continuous casting method is characterized in that the crystallization temperature, which is the temperature at which crystals begin to precipitate, is 500 ° C. or more and 600 ° C. or less.

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