JP3246372B2 - Continuous casting of steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of continuously casting steel to reduce the segregation of cast slabs.

[0002]

2. Description of the Related Art In a continuous casting method of steel, a slab has an unsolidified phase elongated in a casting direction and is drawn out between a number of rolls. The volume of molten steel changes due to solidification and shrinkage. To compensate for the change in the volume of molten steel, concentrated molten steel containing solute elements accumulates at the center of the slab, and the center segregates. Is generated.

[0003] Therefore, in order to reduce the center segregation,
Numerous technologies have been proposed and implemented, such as low-temperature casting and improvement of the casting structure by electromagnetic stirring, and among these many technologies, it has been put to practical use to apply light reduction to unsolidified slabs as an effective means. ing.

[0004] Japanese Patent Publication No. Hei 3-6855 (hereinafter referred to as "prior art 1") discloses a flow limit solid fraction (solid fraction of 0.6 to 0.6) from the time when the center of a slab reaches a liquidus temperature. 0.8)
0.5 mm / min to 2.
It is disclosed that the unsolidified slab is continuously reduced at a rate of 0 mm / min. According to Prior Art 1, the amount of solidification shrinkage can be compensated for by this reduction, and not only the center segregation but also the occurrence of V segregation and reverse V segregation can be prevented.

[0005] Japanese Patent Publication No. 62-34461
According to "prior art 2"), the solidified shell between the mold and the liquidus crater end of the slab is actively bulged to increase the thickness of the unsolidified layer on the slab center side, It is disclosed to apply a reduction to the slab between the liquidus crater end and the solidus crater end. According to Prior Art 2, since the slab is positively bulged in a region where the center segregation does not occur, the unsolidified layer is thickened and then reduced, so that the blockage of the unsolidified layer can be prevented, and the center segregation is greatly reduced. It is possible to reduce it.

[0006]

However, according to the prior art 1, when the slab width is more than 1800 mm, the unsolidified layer has a large slab width due to non-uniform solidification in the slab width direction. Because of the blockage in the direction, it is not possible to greatly improve center segregation over the entire width of the slab. Further, since both short sides of the slab after solidification are also reduced, a large rolling force is required.

In the prior art 2, since the short side of the slab is not reduced and the unsolidified layer at the center of the slab width is thick, the above two problems in the prior art 1 are solved. It is difficult to generate large bulging, that is, it is difficult to secure a large rolling reduction, because the instability of casting due to active bulging and the occurrence of slab internal cracks due to bulging are concerned. Center segregation is not sufficiently improved.

The present invention has been made in view of the above circumstances, and its purpose is to increase the thickness of the unsolidified layer at the center of the slab width at the exit of the mold so that the unsolidified layer is formed near the final solidified portion. It is an object of the present invention to provide a continuous casting method of steel capable of preventing occurrence of blockage, suppressing flow of concentrated molten steel as much as possible, and greatly improving center segregation of a slab.

[0009]

Means for Solving the Problems] continuous casting of steel according to the first invention, the slab shape in cast-type outlet, slab width central portion of the slab thickness as compared to the slab short side and enlarged shape, continuous steel casting the slab by adding roll reduction to the enlarged portion of the slab thickness in the period leading up to completion of solidification in the secondary cooling zone
In the casting method, the slab width is W and the slab thickness at the short side of the slab is
D, distance from meniscus to solidus crater end
L, the casting at the center of the slab width at the mold exit
The expansion amount d of Kataatsumi is characterized in range and be Rukoto of (1) below. d> D (W-2.4D) / 2L (1)

In the present invention, solidification in the mold is advanced by using a mold having a thicker slab thickness on the center side in the slab width direction, and the slab shape at the mold outlet is changed to a shape with a thicker slab thickness on the slab center side. That is, since the unsolidified layer at the center of the slab has a thick shape,
It is not necessary to further perform bulging immediately below the mold, so that instability of casting caused by bulging immediately below the mold and occurrence of internal cracks in the slab due to bulging can be eliminated. Also, by simply changing the shape of the mold, the slab thickness expansion amount can be arbitrarily and increased, so that the occurrence of blockage of the unsolidified layer near the final solidified portion is prevented, and the reduction of the slab is reduced. This can be performed sufficiently, and the effect of improving center segregation is great. Then, in order to prevent the center segregation, it is sufficient to reduce only the enlarged portion of the slab thickness, so that it is possible to reduce with a reduction force slightly larger than the molten steel static pressure.

At this time, the amount of increase in the thickness of the slab is determined by the final solidification level.
Flow of concentrated molten steel in the vicinity of
Its value must be determined with the intention of realizing the situation.
It is necessary.

As shown in FIG . 5, the slab width is W, and the short side of the slab is
Is the slab thickness at D, and the range of the enlarged slab thickness in the slab width direction.
W-2.4D at the center of the slab width at the mold exit
Assuming that the expansion of the slab thickness is d, the unsolidified layer in the slab width direction
The angle φ (radian) is approximated by the following equation (3). φ ≒ 2d / (W-2.4D) (3)

As shown in FIG. 6, the slab at the short side of the slab
Thickness D, from meniscus to solidus crater end
Assuming that the distance is L, the unsolidified layer angle θ in the casting direction (radius
Is approximated by the following equation (4). θ ≒ D / L (4)

Concentration melting in the slab width direction near the final solidification position
In order to make the flow of steel near steady state, that is, solidification
Even when the volume of molten steel changes due to shrinkage, etc.
The steel does not flow and the molten steel with low solid fraction and unconcentrated
In order to replenish the shortage of molten steel volume by flowing, the slab width
The unsolidified layer angle φ in the casting direction from the unsolidified layer angle θ in the casting direction.
It is desirable to make it larger.

The angle φ of the unsolidified layer in the slab width direction is
As a condition for making the angle larger than the unsolidified layer angle θ, the equation (3) and
From equation (4), the slab at the center of the slab width at the exit of the mold
The enlargement amount d of the thickness is derived as the aforementioned equation (1).

The continuous casting method for steel according to the second invention is characterized in that
In the invention, the slab width is W and the slab thickness at the short side of the slab is
D, at least W-2.4 on the slab width center side
Characterized by increasing the slab thickness in the range of D
It is.

In the continuous casting method, various types of
It is necessary to cast the slab width. In the present invention,
Casting is performed using a mold with a thicker slab on the center side.
When the slab thickness is increased over the entire mold width,
The mold must be changed every time the width is changed, which is extremely ineffective
Rate. Therefore, in the present invention, the slab thickness
Provide a flat area and change the slab width in this flat area.
The ability to cast slabs of various widths with the same mold
It was investigated. However, when this flat part becomes wider, the unsolidified layer
Blockage reduces the effect of reducing center segregation due to roll pressure
You. Therefore, the range of the enlarged part of the slab thickness is varied in the slab width direction.
As a result of conducting a survey in comparison with the effect of improving center segregation
When the slab width is W and the slab thickness at the short side of the slab is D,
Enlarged part of slab thickness secures at least W-2.4D range
If it is done, it turns out that the effect of improving center segregation is not impaired
did. That is, the thickness of the slab on the outside may be flat.
I understood. The range of the enlarged part of the slab thickness is W-2.4D
When it is less than, the clogging of the unsolidified layer occurs on the short side of the slab
Reduction of rolling reduction effect and improvement of center segregation on short side of slab
Not enough and not desirable.

In this manner, the shortest side of the cast slab is
Large, flat 1.2 mm length of slab thickness on short side of slab
It is possible to change the slab width in this flat part
Can do it. For example, the slab thickness on the short side is 230
mm with a maximum width of 2150 mm
If it is a mold, the enlarged part of the slab thickness is 1598m on the width center side
m, and a range of 276 mm on one side becomes a flat portion.
The casting with a width of 2150 mm to 1600 mm
Pieces can be cast.

The continuous casting of steel according to the third invention, first
In the invention of the second aspect, the shape of the thickness of the slab at the outlet of the mold in the slab width direction is determined according to the following equation (2). y = K ^{[(x / lo) 2 -2 (} x / lo) 3 + (x / lo) 4 ] ... (2)

In the equation (2), y is the displacement of the slab thickness in the direction of enlargement with respect to the flat surface of the slab thickness, and is equal to 1/2 of the slab thickness enlargement. Is the distance in the slab width direction starting from the position at which the slab thickness started to expand, and lo is the length of the slab thickness expansion portion in the slab width direction, and K is a constant. Then, when the amount of expansion of the slab thickness at 1/2 of the mold width is d, 1/2 of the mold width, ie, x
= Lo / 2, the displacement amount y is 1 of the expansion amount d of the slab thickness.
K is determined as equal to / 2.

In the present invention, since the slab thickness is not constant in the width direction, there is a concern that the slab may be locally deformed by bulging due to the molten steel static pressure immediately below the mold. If local deformation occurs, the slab cracks and operation errors such as breakouts occur.Therefore, the shape of the slab at the exit of the mold will not change even if deformation due to bulging occurs. It is desirable that the shape does not occur. Therefore, if the shape at the time of bulging immediately below the mold is given in advance, the deformation due to bulging becomes uniform in the slab width direction, and local deformation can be prevented. The curve closest to the bulging shape of the slab is the deformation curve of the beam when an equal load is applied to the beam fixed at both ends, so the deformation curve of the beam when the equal load is applied to the beam fixed at both ends, that is,
Equation (2) is adopted to determine the shape of the slab at the exit of the mold.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 is a schematic view of a side section at the center position of a slab width of a continuous caster having a rectangular slab section to which the present invention is applied, and FIG. 3 is shown.

As shown in FIG. 3, the mold 1 used in the present invention
Is a pair of opposed mold long sides 15 of copper and water-cooled structure,
It is composed of a pair of opposed short sides 16 of a water cooling structure made of copper sandwiched between the long sides 15 of the mold. The distance between the opposed mold long sides 15 is such that the center of the width is the mold short side 16.
It is wider than the side, and forms an enlarged portion 17 of the slab thickness, and the outside is a flat portion 18, 18a in which the distance between the opposing mold long sides 15 is constant. The mold short side 16 slides in the mold long side 15 only at the flat portions 18 and 18a,
The slab width (W) can be changed. The slab thickness is
The slab thickness gradually increases from the slab thickness (D) on the short side 16 of the mold, and the slab thickness (D +
d), and the slab thickness is substantially symmetrical with respect to the center line of the long side 15 of the mold.

As shown in FIG. 2, the molten steel 11 in the tundish 12 is provided at the bottom of the tundish 12 and has a tip dipped in a meniscus 14.
Through the mold 1. The molten steel 11 injected into the mold 1 is cooled by contacting the mold 1 to form a solidified layer 7 on the outer periphery. Then, the solidified layer 7 is provided on the support roll 3 and the guide roll provided in the secondary cooling zone 2 below the mold 1. 4, and is continuously pulled downward through the reduction roll 5.
The secondary cooling zone 2 is a cooling zone by water spray or air mist spray. The surface of the solidified layer 7 drawn from the mold 1 is cooled, and the thickness of the unsolidified layer 8 inside the solidified phase 7 is reduced. The solidification is completed at the solidus crater end 10 to form a slab 6. The dashed line shown in FIG. 2 is the liquidus temperature, and the solid phase appears at the center of the slab thickness at the tip position of the liquidus temperature, that is, below the liquidus crater end 9 in the drawing direction. It becomes a coexistence layer.

During the period until the solidification during the drawing in the secondary cooling zone 2 is completed, the reduction roll 5
Roll pressure is applied with. At this time, it is preferable to reduce the slab thickness until the slab thickness on the slab center side and the slab short side side becomes substantially the same, because handling of the slab 6 to the next step becomes easy. FIG. 4 schematically shows a shape change of the slab 6 in the secondary cooling zone 2 due to a roll reduction, and FIGS. 4 (a), (b), (c) and (d) are diagrams respectively. II, II-II, III-III, and IV-
Corresponds to IV section.

The roll reduction is such that the solid phase ratio at the center of the slab thickness at the center position of the slab width is in the range of 0.01 to 0.75,
It is desirable to carry out continuously at a rate of 0.5 mm / min to 2.5 mm / min.

If the solid phase ratio at the center of the slab thickness at the center of the slab width is less than 0.01, the molten steel 11 flows easily, and the center segregation does not occur. If the solid phase ratio at the center of the slab thickness exceeds 0.75, the flow of the unsolidified layer 8 in the solid-liquid coexisting layer becomes impossible, and the unsolidified layer 8 does not move even if the roll is rolled down.
This is because there is no effect of rolling down.

The speed under roll pressure is 0.5 mm / min.
If it is less than 30 mm, the effect of reducing center segregation cannot be sufficiently obtained because the amount of reduction is insufficient, and if it exceeds 2.5 mm / min, the deformation of the solidified layer 7 becomes large and internal cracks occur.

FIG. 1 shows the case where the present invention is applied.
An enlarged portion 17 of the slab thickness and flat portions 18 and 18 on the short side of the slab.
3A shows an example of the transition of the slab thickness at a distance from the meniscus 14. Expanded part 1 of slab thickness
Rolls 7 are continuously rolled down in the solid phase ratio of 0.01 to 0.75, and at the position of the solid phase ratio of 0.75, the slab 6 has a substantially flat shape over the entire width. In FIG. 1, the thickness of the slab on the short side of the slab also decreases continuously, because the thickness of the slab shrinks due to the volume shrinkage accompanying the temperature decrease after solidification.

The length (lo) in the width direction of the enlarged portion 17 of the slab thickness
It is preferable that, when the slab width is W and the slab thickness at the short side of the slab is D, at least the range of W-2.4D at the center of the slab width is secured. Therefore, in the mold 1, if the range of (maximum width-2.4 x slab thickness) is defined as the enlarged portion 17 of the slab thickness with respect to the maximum width that can be cast, the slab width in the mold 1 is the maximum width. From (maximum width-2.4 x slab thickness). If there is a large difference in casting width and one casting mold 1 cannot handle all casting widths, a plurality of casting molds 1 having different maximum widths may be prepared as described above.

The expansion amount (d) of the slab thickness at the width center position of the slab 6 is determined by the slab width (W), the slab thickness (D) at the short side of the slab, and the solidus line from the meniscus 14. From the distance (L) to the crater end 10, the right side of the equation (1) is calculated,
It is desirable that the value be larger than the calculated value. Here, the distance (L) from the meniscus 14 to the solidus crater end 10 can be accurately obtained by calculating and calculating the solidification coefficient each time based on the cooling strength of the secondary cooling zone 2. The unsolidified layer angle θ is larger when the secondary cooling strength is high and covers the range where the secondary cooling strength is low, so that the solidification coefficient when the secondary cooling strength is high (about 30 mm
/ Min ^1/2 ) may be used.

The upper limit of the amount of increase (d) in the thickness of the slab may be determined within a practical range in consideration of economy, as long as no internal cracking occurs due to the subsequent roll reduction.

The shape of the slab thickness in the slab width direction at the mold outlet is desirably determined according to the equation (2). In this case, the slab thickness expansion amount (d) at the center position of the slab width and the length (lo) of the slab thickness expansion portion 17 in the casting width direction are determined in advance, and the equation (2) is used. y = d / 2, x = lo
/ 2 is substituted to determine a constant K, and then the displacement (y) at an arbitrary position (x) is calculated to determine the shape in the slab width direction.

After leaving the mold 1, it may be further bulged before starting the reduction, and then reduced.

[0035]

EXAMPLE The present invention was carried out using a continuous casting machine shown in FIG. 2 and a mold shown in FIG. The target steel type is C: 0.15wt
%, Si: 0.2 wt%, and Mn: 0.5 wt%, a 40 kg class Al-killed steel, the casting speed was 1.8 m / min, and the degree of superheat of the molten steel in the tundish was 25 to 35 ° C.

The mold used in the present invention has a slab thickness of 230 mm on the short side and a maximum castable width of 2150 mm. Then, the minimum width of the enlarged portion of the slab thickness is calculated by the equation (5), and from the result, the minimum width of the slab 16
The area of 00 mm is defined as an enlarged part of the slab thickness, and 2150 m
Cast pieces of m width and 1600 mm width were cast. 2150−2.4 × 230 = 1598 (mm) …… (5)

Next, the distance (L) from the meniscus to the solidus crater end was determined by the solidification coefficient = 30 mm / min.
^{Using 1/2} , it was calculated by equation (6), and from the calculated distance (L), the right side of equation (1) was calculated by equation (7), and the enlargement amount (d) of the slab thickness was determined to be 20 mm. . L = [(230 / (2 x 30)] ² x 1800 = 26450 (mm) ... (6) d> [230 x (2150-2.4 x 230)] / (2 x 26450) = 6.9 (mm) ... … (7)

In the equation (2), lo = 1600 m
Assuming that the displacement amount (y) at m, x = 800 mm is 10 mm, a constant K = 160 mm is obtained, the displacement amount (y) at an arbitrary position (x) is calculated, the shape of the slab thickness is determined, and along the shape, The long side of the mold was processed.

Rolling down to the portion where the slab thickness is increased starts from the position where the solid fraction at the center of the slab thickness at the center of the width is estimated to be 0.05, and the solid fraction is estimated to be 0.75. And the rolling speed during this time is 1.8 mm / min
Is set so as to reduce the roll interval of the reduction roll at a fixed rate so that the thickness of the slab becomes substantially flat. The rolling load of the rolling roll at this time was 62 tons, 97 tons, and 123 tons per rolling roll.
The test was conducted at three levels of tons, and the effect of the rolling load was also investigated. still,
When a roll reaction force equal to or greater than the set value is applied, the rolling roll retreats to the balance position.

The cast slab is 1600 mm wide and 21 mm wide.
In any of the 50 mm widths, the liquid phase is released in the slab width direction until near the completion of solidification, so that steady flow of molten steel is ensured and accumulation of concentrated molten steel does not occur, so that the degree of occurrence of center segregation and center porosity is reduced. Significant improvement over the entire slab width. The effect of the rolling load on the occurrence of center segregation and center porosity is as follows:
A sufficient effect was recognized at tons, and in the case of 123 tons, a slight decrease in the slab thickness was recognized, and it is estimated that the slab was reduced even after the slab was completely flattened. By controlling the rolling load per pair of rolling rolls to the optimum value in this way, it is possible to accurately reduce the desired solid phase ratio range, and to avoid unnecessary reduction immediately before final solidification.

[0041]

According to the present invention, it is possible to increase the thickness of the slab at the center of the slab without having to worry about instability of casting or the occurrence of internal cracks in the slab due to bulging. By increasing the thickness of the unsolidified layer at the center of the width,
Prevents clogging of the unsolidified layer in the vicinity of the final solidification zone and generates a steady flow of molten steel to suppress the flow of concentrated molten steel as much as possible. it can.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of transition of a slab thickness in an enlarged portion of a slab thickness on a slab width center side and a flat portion on a short side of a slab when the present invention is applied.

FIG. 2 is a schematic view of a side cross section at a center position of a slab width of a continuous casting machine having a rectangular slab section to which the present invention is applied.

FIG. 3 is a schematic view of a plane cross section of a mold to which the present invention is applied.

FIG. 4 is a view schematically showing a change in the shape of a slab due to roll reduction in a secondary cooling zone when the present invention is applied, and (a), (b), (c), and (d). Are respectively II section, II-II section, III-III section, and IV section in FIG.
-Corresponds to section IV.

FIG. 5 is a diagram schematically showing an unsolidified layer angle φ in a slab width direction.

FIG. 6 is a view schematically showing an unsolidified layer angle θ in a casting direction.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 mold 2 secondary cooling zone 3 support roll 4 guide roll 5 reduction roll 6 cast piece 7 solidified layer 8 unsolidified layer 9 liquidus crater end 10 solidus crater end 11 molten steel

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-5-185186 (JP, A) JP-A-3-32455 (JP, A) JP-A-6-262318 (JP, A) JP-A-6-262318 154979 (JP, A) JP-A-6-154980 (JP, A) JP-A-62-233048 (JP, A) JP-A-60-6254 (JP, A) JP-A 9-1292 (JP, A) JP-A-8-215815 (JP, A) JP-A-2-15858 (JP, A) JP-A-2-500501 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/04 311 B22D 11/128 350 B22D 11/20

Claims (3)

(57) [Claims] The method according to claim 1] billet in cast-type outlet shape, a shape slab width central portion of the slab thickness is enlarged compared to the slab short side, it casts the period until completion of solidification in the secondary cooling zone Continuous casting of steel by casting a slab by applying roll reduction to the enlarged part of the piece thickness
In the casting method, the slab width is W and the slab thickness at the short side of the slab is
D, distance from meniscus to solidus crater end
L, the casting at the center of the slab width at the mold exit
Continuous casting of steel characterized by larger amounts d the range and be Rukoto of (1) below the Kataatsumi. d> D (W-2.4D) / 2L (1) 2. The slab width is W and the slab thickness at the short side of the slab is D.
And at least W-2.4D at the center of the slab width
The method for continuously casting steel according to claim 1, wherein the thickness of the slab is increased in the range of (1). 3. A slab width direction of a slab thickness at a mold outlet.
Is determined in accordance with the following equation (2).
The method for continuously casting steel according to claim 1 or 2. y = K ^{[(x / lo) 2 -2 (} x / lo) 3 + (x / lo) 4 ] ... (2) However, y in equation (2), based on the flat surface of the slab thickness
The displacement of the slab thickness in the direction of enlargement
X is equal to 1/2 of the expansion amount, x is the slab thickness start expanding
Distance in the slab width direction from the starting position, and lo
Is the length in the slab width direction of the enlarged portion of the slab thickness, and
K is a constant, the amount of increase in slab thickness at half the mold width
Where d is half of the mold width, ie, x = lo / 2.
The displacement y is equal to 1/2 of the expansion d of the slab thickness.
Is determined as K.