JP2003326344A - Method for continuously casting cast bloom - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent occurrence of skull on molten steel surface and deterioration of center segregation by restraining lowering of temperature on the molten steel surface in a mold and flowing-out of the high temperature molten steel into the lower part of the mold when a bloom is continuously cast. <P>SOLUTION: When the bloom is continuously cast, a cast condition is adjusted so as to satisfy a relation of the following formula: 0.0002<[(D/2)×ω×(Bmax2-Bs2)/Ls]0.5/[(Q/S)×sinθ]<0.02. Wherein, Q: molten steel supplying speed (ton/min), S: total area (m<SP>2</SP>) of spouting holes in an immersion nozzle, θ: spouting angle (°) of the spouting hole in the immersion nozzle, D: average length (m) of the long side surface and short side surface in the mold, Ls: distance (m) from the molten metal surface to the maximum magnetic flux generating position, Bmax: the maximum magnetic flux density (T) as the front face of an electromagnetic stirring apparatus in the mold, Bs: the magnetic flux density (T) on the molten steel surface in the mold and ω: angle speed (rad/s) of the magnetic field for stirring. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a continuous casting method for bloom slabs.

[0002]

2. Description of the Related Art In continuous casting of steel, oxide flux (mold powder) is generally used as a lubricant between a mold and a slab, but when the flow velocity near the molten metal surface in the mold is increased, molten steel entrains the flux, It is known that defects are generated in the slab by being trapped in the solidified shell.

Therefore, in order to reduce the flow velocity near the molten metal surface, there is a method in which the discharge hole of the dipping nozzle for supplying molten steel into the mold is directed downward so that the influence of the discharge flow does not reach near the molten metal surface. It is generally performed (for example, see Patent Document 1).

[0004]

[Patent Document 1] JP-A-63-188459

[0005]

However, in the method as disclosed in the above publication, the high temperature molten steel supplied from the tundish through the immersion nozzle becomes difficult to be supplied to the molten metal surface, so that the temperature of the molten metal surface Deteriorates to generate metal, which often precipitates together with the mold flux to cause similar defects, or insufficient melting of the mold flux causes defects on the slab surface. Therefore, under such conditions, it is unavoidable to raise the temperature of the molten steel in the tundish before it is poured into the mold, but the fact that the discharge hole of the immersion nozzle is facing downwards also overlaps, and the molten steel at a higher temperature falls below the mold. Since the conditions are such that the slab easily flows out, the solidification structure in the central part of the slab changes from an equiaxed crystal to a columnar crystal, causing a problem that center segregation deteriorates. Further, the flow of the molten steel to the lower part of the mold becomes remarkable, so that the floating ability of the non-metallic inclusions in the mold is impaired and the defects of inclusion physical properties increase.

As another method, in order to solve the above-mentioned problems, a technique for uniforming the temperature of the molten metal surface and preventing the generation of metal by applying an alternating magnetic field to the molten steel in the mold in the horizontal direction and electromagnetically stirring the molten steel. Is generally known. However, even with this method, it is not possible to raise the temperature of the hot water surface sufficiently,
On the contrary, if the mixture is agitated too much, the mixing with the low temperature molten steel in the lower part of the mold may be accelerated to lower the molten metal surface temperature, or the molten steel temperature in the lower part of the mold may be increased, and the center segregation may be deteriorated. There was a problem that it was not always possible to obtain a sufficient effect.

The present invention has been made to solve the above-mentioned conventional problems. When the bloom is continuously cast, the temperature drop of the molten metal in the mold is suppressed, and the outflow of the high temperature molten steel to the lower part of the mold is suppressed. An object of the present invention is to provide a continuous casting method for bloom slabs that can prevent the generation of metal on the molten metal surface and the deterioration of center segregation by suppressing the above.

[0008]

According to a first aspect of the present invention, continuous bloom slab is provided in which the molten steel in the mold is electromagnetically stirred so as to horizontally swirl while supplying the molten steel from the tundish to the mold through the immersion nozzle. In the casting method, by adjusting the casting conditions so that the ratio of the vertical gradient of the magnetic field applied to horizontally swirl the molten steel and the downward velocity component of the molten steel discharged from the immersion nozzle is within a predetermined range. The above-mentioned problems are solved.

According to a second aspect of the present invention, in the continuous casting method for bloom slabs according to the first aspect, the casting condition is that the vertical gradient of the magnetic field below the maximum magnetic flux generation position is above and below the magnetic field of information. The above problem is similarly solved by adjusting the casting conditions so as to be gentler than the directional gradient.

According to a third aspect of the present invention, a maximum magnetic flux is generated in a continuous casting method of bloom slabs in which molten steel is supplied from a tundish to a mold through an immersion nozzle and electromagnetically stirred so as to horizontally swirl the molten steel in the mold. The above problems are similarly solved by adjusting the casting conditions so that the vertical gradient of the magnetic field below the position is less than the vertical gradient of the upper magnetic field.

According to a fourth aspect of the present invention, in the continuous casting method for bloom slabs according to the first or third aspect, the casting condition is 0.0002 <{(D / 2) .ω. (Bmax ² − Bs ² ) / Ls} ^0.5 ÷ {(Q / S) · sin θ} <0.02 (1) (where, Q: molten steel supply rate (ton / min) S: total area of immersion nozzle discharge holes (m ² ) θ: Discharge angle of dipping nozzle discharge hole (°) D: Average length of long and short sides of the mold (m) Ls: Distance from molten metal surface to maximum magnetic flux generation position (m) Bmax: Within mold The maximum magnetic flux density (T) on the front surface of the electromagnetic stirrer Bs: the magnetic flux density (T) on the molten metal surface in the mold ω: the angular velocity (rad / s) of the magnetic field for stirring is satisfied.

According to the invention of claim 5, in the continuous casting method for bloom slabs according to claim 2 or 3, the casting conditions are as follows: {(Bmax ² −Bu ² ) / Lu} ^0.5 / {(Bmax ² -Bs ² ) /
Ls} ^0.5 <1 (where, Ls: distance from the molten metal surface to the maximum magnetic flux generation position (m) Lu: below the maximum magnetic flux density generation position and the magnetic flux density is √ (1/2 · Bmax ² ). (Max) Bmax: Maximum magnetic flux density (T) in front of the electromagnetic stirrer in the mold Bs: Magnetic flux density (T) at the molten metal surface in the mold Bu: Bu = √ (1/2 · Bmax ² )).

According to a sixth aspect of the present invention, in the continuous casting method for bloom slabs according to the first or second aspect, the superheat degree of the molten steel in the tundish supplied to the mold is 10 ° C to
The temperature is maintained in the range of 30 ° C.

According to a seventh aspect of the present invention, in the continuous casting method for bloom cast pieces according to any of the first to third aspects, the discharge angle θ of the immersion nozzle discharge hole is extended from the discharge hole in the discharge direction. The intersection of the line and the inner surface of the mold is adjusted to be above the maximum magnetic flux generation position.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

First, an embodiment according to the invention of claim 1 will be described. In the first embodiment, while the molten steel is supplied from the tundish to the mold through the immersion nozzle, the molten steel in the mold is horizontally swirled while electromagnetically stirring the molten steel so as to horizontally swirl the molten steel. The casting conditions are adjusted so that the ratio of the vertical gradient of the magnetic field applied for this purpose and the lower velocity component of the molten steel discharged from the immersion nozzle falls within a predetermined range. Specifically, the casting conditions are set so as to satisfy the relationship (1).

First of all, the knowledge which is the basis of the present invention will be described. FIG. 1 (A) shows the relationship between the molten steel 1 that is solidifying in the mold and the electromagnetic stirrer 2 composed of an alternating-current electromagnetic coil arranged around the outside of the mold when viewed from diagonally above the mold. It is shown by the schematic diagram of
FIG. 3B shows the distribution of the magnetic flux density B by the electromagnetic stirring device 2 in correspondence with the height of the mold. However,
For clarity, the mold is omitted in this figure.

In the figure, a solid arrow 3 indicates a horizontal swirl flow of molten steel 1 generated by the electromagnetic stirrer 2 (here, a clockwise direction), and a broken arrow 4 indicates molten steel 1 induced by the horizontal swirl flow 3. The vertical swirl flow (described later), reference numeral 5 is a cast piece in which the surface of the molten steel 1 is solidifying, 6 is the molten metal surface of the molten steel in the mold, and 7 is an immersion nozzle.

As shown in FIG. 1, when the molten steel 1 in the mold is horizontally rotated by the electromagnetic stirrer 2 to generate the swirling flow 3 for stirring, the volume force generated by the rotating magnetic field is approximately as follows. It is known to be expressed by

F = 0.5 · σ · r · ω · B ² (3)

Here, F is the volume force acting on the molten steel, σ is the electrical conductivity of the molten steel, r is the distance from the center of the rotating magnetic field, ω is the rotational velocity (angular velocity) of the rotating magnetic field, and B is the magnetic flux density.

In this case, if there is a difference in the magnetic flux density in the casting direction (vertical direction), a pressure gradient is generated along the wall surface of the mold, so that the magnetic flux density is Bmax above the approximate coil center position of the electromagnetic stirrer 2. Then, the secondary vertical swirling flow 4 is generated in the vicinity of the wall surface of the mold, and the flow velocity becomes more remarkable as the gradient of the volume force F increases.

By the way, the downward velocity component (downward velocity component) of the molten steel discharge flow discharged from the immersion nozzle 7 into the mold.
On the other hand, when the secondary upward flow 4 is small, the injection flow of the high-temperature molten steel supplied from the immersion nozzle 7 flows out below the mold, so that the high-temperature molten steel flows on the molten metal surface. It becomes difficult to be supplied, and as a result, the temperature of the molten metal surface is significantly lowered.

On the other hand, if the upward flow 4 generated secondarily is too large with respect to the downward velocity component of the molten steel discharge flow from the dipping nozzle 7, a large amount of low-temperature molten steel at the bottom of the mold is also near the molten metal surface. However, the temperature of the surface of the molten metal is lowered.

The inventors of the present invention have paid attention to the above-mentioned phenomenon, and as a result of detailed investigation and examination, as a result, the strength of the ascending flow 4 along the secondary mold wall surface generated by the electromagnetic stirring and the supply from the dipping nozzle 7 It has been found that the temperature of the molten metal surface rises when the strength of the downward injection flow is well balanced, that is, when the relationship of the expression (1) is satisfied, and can be maintained in a suitable range.

FIG. 2 shows the results of an examination of molten steel in the tundish having a superheat degree (difference between molten steel temperature and solidification temperature) of 25 ° C., which is the basis of this finding, when continuous casting is performed under different casting conditions. . In FIG. 2, the horizontal axis represents the calculation result by the above formula (1), and the vertical axis represents the molten steel superheat degree of the molten metal surface in the mold obtained by the ordinary method. From this figure, it can be seen that the temperature of the molten metal surface remarkably rises when the casting conditions satisfy the relational expression (1).

At that time, how to discharge the molten steel from the immersion nozzle 7 is also important. When the injection amount of the high temperature supplied from the nozzle 7 enters below the maximum magnetic flux density Bmax generation position (substantially the coil center position) of the electromagnetic stirring device 2, the downward flow generated by the pressure gradient in the opposite direction. The reason for this is that you will get on board 9 and flow downward.

Therefore, the discharge angle (inclination from the horizontal direction) θ of the immersion nozzle 7 is set in consideration of the height of the discharge hole of the nozzle 7, and as shown schematically in FIG. It is desirable to set the intersection of the extension line extending in the (angle θ direction) and the inner surface of the mold 8 so as to be above the position where the maximum magnetic flux density Bmax of the electromagnetic stirring device 2 is generated. FIG. 1
In the case of, it corresponds to setting so as to enter above the position of the distance Ls from the bath surface 3.

[0029]

[Embodiment 1] Next, the results of examining the degree of superheat of molten steel in a tundish before being poured into a mold will be described. Specifically, continuous casting of steel was carried out under the following basic conditions, and it was investigated whether or not product defects (inclusion physical property defects, etc.) occurred after rolling the bloom into bar steel. The result is shown in FIG. In this figure, the horizontal axis is the same as in the case of FIG. 2, but the vertical axis is the degree of superheat of molten steel in the tundish.

<Basic conditions> ・ Mold size 300 mm × 400 mm (D = 0.35)
m) -Mold speed 0.4 to 1.3 m / min-Molten steel injection speed Q 0.34 to 1.10 ton / min-Immersion nozzle discharge hole 40 mmφ x 4 holes (S = 0.00
502m ² ) ・ Immersion nozzle discharge angle θ downward 5-60 ° ・ Maximum magnetic flux density 0.020-0.045T ・ Magnetic field rotation speed 8π rad ・ Maximum magnetic flux density generation position Bmax 0.35m below the molten metal surface ・ On the molten metal surface Magnetic flux density of Bs 0.6 × BmaxT ・ Temperature of molten steel 7-39 ℃

From the defect occurrence situation of the steel bar shown in FIG. 4, defects (inclusional property defects, inclusion defects, when the superheat degree of molten steel in the tundish is 30 ° C. or less under the condition that the above expression (1) is satisfied. It can be seen that the occurrence of central segregation defects) is significantly reduced. On the other hand, from this experiment, 7 ° C was also good, but if the temperature of the molten steel injected into the mold is too low, the raising effect will be small, so the heating degree of the molten steel temperature in the tundish should be 10 ° C or higher. Is desirable.

According to the present embodiment described in detail above, it is possible to suppress the lowering of the molten metal surface temperature and perform casting at a high temperature, and it is also possible to suppress the molten steel flow below the mold. It became possible to manufacture a sound slab with few center segregation defects.

Next, a second embodiment according to the invention of claim 2 will be described in detail.

First, the findings that have been obtained by various investigations to further improve the quality of the slab obtained by continuous casting and are the basis of the present invention will be described.

As described above, when the molten steel in the mold is rotationally stirred in the horizontal direction, the rotating magnetic field causes the volume force represented by the above formula (3). In that case, if there is a difference in the magnetic flux density in the casting direction (vertical direction), a pressure gradient is generated along the wall surface of the mold, so that as shown in FIG. An upward secondary swirl flow 4 and a downward swirl flow 9 near the wall surface of the mold are generated in the lower part of the electromagnetic stirring device 2, respectively. 4, 9 are the above (3)
The larger the gradient of the volume force F obtained by the equation, the more remarkable the flow.

Naturally, in order to prevent the decrease in the molten metal surface temperature, it is important to sufficiently supply the high temperature molten steel injected from the dipping nozzle 7 to the molten metal surface.
It has been found that the effect of increasing the surface temperature of the molten metal is not only dependent solely on the horizontal stirring strength, but is significantly affected by the secondary swirling flows 4 and 9 in the upper and lower stages.

Then, whilst maintaining the condition that the high-temperature molten steel stays at the upper part of the mold to some extent, the lower secondary swirling flow 9 is suppressed, that is, as in the invention of claim 3,
It is important to make the magnetic field gradient in the lower stage gentler than that in the upper stage. Specifically, by adjusting the casting conditions so as to satisfy the relationship of the above formula (2), the high temperature molten steel is placed below the mold. It was found that since outflow can be suppressed, the formation of equiaxed crystals is promoted during solidification, and the deterioration of center segregation can be prevented.

As a result of further detailed examination, it is more effective to use the invention of claim 1 in combination, that is, the invention of claim 2, in order to sufficiently supply the high temperature molten steel to the molten metal surface. It was discovered.

The present embodiment is based on this finding, realizes the casting conditions in the first embodiment, and further, the vertical gradient of the magnetic field below the maximum magnetic flux generation position is the vertical gradient of the upper magnetic field. The casting conditions are adjusted so that it becomes looser.

Specifically, when continuously casting molten steel while horizontally swirling by electromagnetic stirring, the following two conditional expressions to be re-posted: 0.0002 <{(D / 2) · ω · (Bmax ² −Bs ² ) / Ls} ^0.5 ÷ {(Q / S) · sin θ} <0.02 (1) {(Bmax ² −Bu ² ) / Lu} ^0.5 / {(Bmax ² −Bs ² ) / Ls} ^0.5 < 1 (2) (where, Q: molten steel feed rate (ton / min) S: total area of immersion nozzle discharge holes (m ² ) θ: discharge angle of immersion nozzle discharge holes (°) D: long side of mold And the average length of the short side (m) Ls: Distance from the molten metal surface to the maximum magnetic flux generation position (m) Lu: Below the maximum magnetic flux density generation position and the magnetic flux density is √ (1/2 · Bmax ² ) (M) Bmax: maximum magnetic flux density (T) in front of the electromagnetic stirrer in the mold Bs: magnetic flux density (T) in the molten metal surface in the mold Bu: Bu = √ (1/2 · Bmax ² ) ω: angular velocity (rad / s) of the magnetic field for stirring The casting conditions are adjusted so as to simultaneously satisfy the relationship.

FIG. 5 shows the characteristics of this embodiment shown in FIG.
It is a schematic diagram corresponding to, and the meanings of the reference numerals and symbols used 1 to 6 are the same. In addition, in FIG. 1 (B),
For the sake of convenience, the horizontal axis is shown in association with the first power of the magnetic flux density, but it is actually a scale corresponding to the second power as in FIG.

In this embodiment, the relationship of the above equation (1) is satisfied, and the maximum magnetic flux density is as shown in the distribution state of the vertical magnetic flux density in FIG. 5 (B). The vertical gradient of the magnetic field below the generation position (corresponding to the numerator of equation (2)) is adjusted to be more gentle than the vertical gradient of the magnetic field above (corresponding to the denominator of equation (2)).

[0043]

Example 2 Next, the results of investigating the degree of product defect (inclusion property defect) and center segregation after continuous casting of steel under the following basic conditions and rolling of bloom into bar steel will be described. <Basic conditions> -Mold size 300 mm x 400 mm (D = 0.35 m) -Casting speed 0.4 to 1.3 m / min-Molten steel injection speed Q 0.34 to 1.10 ton / min-Immersion nozzle discharge hole 40 mmφ x 4 holes (S = 0.005
02m ² ) ・ Dip nozzle discharge angle θ downward 5-60 ° ・ Maximum magnetic flux density Bmax 0.020-0.060T ・ Magnetic field rotation speed ω 8π rad ・ Distance from the molten metal surface to the coil center (maximum magnetic flux density generation position) Ls 0.35 m ・ Magnetic flux density on the molten metal surface Bs 0.6 × Bmax T ・ Distance down from the center of the coil to the position where the magnetic flux density is halved Lu 0.18 to 1.64 m (magnetic material below the magnetic stirring device) Install and adjust) ・ Melting steel superheat in tundish 7-30 ℃

FIG. 6 shows the results of investigation of the molten steel temperature in the mold. In this figure, the horizontal axis is the equation (1) and the vertical axis is the equation (2).
Corresponds to the expression. From this figure, it is understood that the temperature of the molten metal surface rises under the condition that the above expressions (1) and (2) are satisfied.

FIG. 7 and FIG. 8 show the occurrence of defects in the steel bar. The relationship between the vertical axis and the horizontal axis is the same as in FIG. From these figures, it is understood that not only the occurrence of inclusion defects is significantly reduced, but also the center segregation is significantly improved under the conditions satisfying the expressions (1) and (2).

According to the present embodiment described in detail above, as in the case of the first embodiment, the ascending swirl generated secondarily in the upper stage by vigorous stirring to the extent that the above equation (1) is satisfied. Since the flow 4 can be made to have an appropriate strength, the decrease in the molten metal surface temperature can be suppressed, and the downward swirling flow 9 generated in the lower stage can be sufficiently suppressed, so that the outflow of the high temperature molten steel to the lower part of the mold is prevented. become able to. Therefore, it is possible to manufacture a higher quality cast piece.

Although the present invention has been specifically described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the invention.

For example, the invention of claim 3 is not limited to the invention used in combination with the invention of claim 1 as in claim 2.

[0049]

As described above, according to the present invention,
When continuously casting blooms, it suppresses the temperature drop of the molten metal surface in the mold, suppresses the outflow of high temperature molten steel to the lower part of the mold, and effectively prevents the generation of metal on the molten metal surface and the deterioration of center segregation. can do.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view schematically showing a phenomenon of flow in a mold when a rotating magnetic field is applied.

FIG. 2 is a diagram showing the relationship between the casting conditions and the molten metal surface temperature in the mold, which is the basis of the invention of claim 1.

FIG. 3 is a schematic diagram for explaining a range of discharge angles of discharge holes of the immersion nozzle.

FIG. 4 is a diagram showing experimental results on molten steel temperature in a tundish and occurrence of product defects which are the basis of the invention of claim 1.

FIG. 5 is a schematic explanatory view schematically showing a flow phenomenon in a mold when the inventions of claims 2 and 3 are applied.

FIG. 6 is a diagram showing the relationship between the casting conditions and the molten metal surface temperature in the mold, which is the basis of the invention of claim 2.

FIG. 7 is a diagram showing the experimental results on the casting conditions and the occurrence of product defects, which are the basis of the invention of claim 2.

FIG. 8 is a diagram showing experimental results on casting conditions and slab center segregation which are the basis of the invention of claim 2.

[Explanation of symbols]

1 ... Molten steel 2 ... Electromagnetic stirrer 3 ... Horizontal swirling flow 4 ... Secondary vertical swirling flow 5 ... Slab 6 ... The surface of the molten metal in the mold 7 ... Immersion nozzle 8 ... Mold 9 ... Secondary vertical downward swirling flow

Claims (7)

[Claims] 1. A continuous casting method of bloom cast slab in which molten steel is supplied from a tundish to a mold through a dipping nozzle while electromagnetic stirring is performed so that the molten steel in the mold is horizontally swirled. A continuous casting method for bloom slabs, characterized in that the casting conditions are adjusted so that the ratio of the vertical gradient of the magnetic field to the vertical velocity component of the molten steel discharged from the immersion nozzle is within a predetermined range. 2. The casting condition is further adjusted so that the vertical gradient of the magnetic field below the maximum magnetic flux generation position is less than the vertical gradient of the upper magnetic field. 2. The continuous casting method for bloom slabs according to 1. 3. A magnetic field below a maximum magnetic flux generation position in a continuous casting method of bloom slabs in which molten steel is supplied from a tundish to a mold through a dipping nozzle while electromagnetic stirring is performed so as to horizontally swirl the molten steel in the mold. The continuous casting method for bloom cast slabs according to claim 1, wherein the casting conditions are adjusted so that the vertical gradient of is lower than the vertical gradient of the upper magnetic field. 4. The casting conditions are as follows: 0.0002 <{(D / 2) ω (Bmax ² -Bs ² ) /
Ls} ^0.5 ÷ {(Q / S) ・ sin θ} <0.02 (where Q: molten steel feed rate (ton / min) S: total area of immersion nozzle discharge hole (m ² ) θ: immersion nozzle discharge hole Discharge angle (°) D: Average length of long and short sides of the mold (m) Ls: Distance from the molten metal surface to the position of maximum magnetic flux (m) Bmax: Maximum magnetic flux in front of the electromagnetic stirrer in the mold The density (T) Bs: the magnetic flux density (T) at the molten metal surface in the mold (ω): the rotational speed (rad / s) of the magnetic field for agitation). Continuous casting method for bloom slabs. Wherein said casting conditions, the following formula ^{{(Bmax 2 -Bu 2) /} Lu} 0.5 / {(Bmax 2 -Bs 2) /
Ls} ^0.5 <1 (where, Ls: distance from the molten metal surface to the maximum magnetic flux generation position (m) Lu: below the maximum magnetic flux density generation position and the magnetic flux density is √ (1/2 · Bmax ² ). (Max) Bmax: Maximum magnetic flux density (T) in front of the electromagnetic stirrer in the mold Bs: Magnetic flux density (T) at the molten metal surface in the mold Bu: Bu = √ (1/2 · Bmax ² )) Is satisfied, the continuous casting method for bloom slabs according to claim 2 or 3, wherein 6. The continuous casting method for bloom cast slabs according to claim 1, wherein the degree of superheat of the molten steel in the tundish supplied to the mold is maintained in the range of 10 ° C. to 30 ° C. 7. The height and discharge angle θ of the immersion nozzle discharge hole are set so that the intersection of the extension line extending from the discharge hole in the discharge direction and the inner surface of the mold is above the maximum magnetic flux generation position. The continuous casting method for bloom slab according to any one of claims 1 to 3, characterized in that.