JP3114671B2 - Steel continuous casting method - 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 for reducing segregation occurring at the center of a continuously cast slab of steel and for preventing internal cracks in the slab.

[0002]

2. Description of the Related Art In the production of cast slabs by continuous casting, an internal defect called center segregation occurs, which is a problem. This center segregation is caused by C and C at the center in the thickness direction, which is the final solidification part of the slab.
This is a phenomenon in which molten steel components such as S, P, and Mn are concentrated and segregated in the positive direction, which causes a decrease in toughness of a steel material and causes hydrogen-induced cracking.

The following two points are considered as causes of the center segregation. (1) In the final stage of solidification of a slab, residual molten steel in which molten steel components such as C, S, P and Mn are concentrated between dendrites, which is one of the solidified structures, is caused by solidification shrinkage and bulging. Moving macroscopically toward the solidification completion point of the final solidification part. (2) The molten steel component is concentrated in the center of the slab in the thickness direction and solidifies as it is.

[0004] Therefore, in order to prevent center segregation, it is considered effective to prevent the movement of molten steel remaining between dendrites and to prevent local accumulation of concentrated molten steel. Such a technique is disclosed.

For example, Japanese Patent Application Laid-Open No. 63-252655 discloses that the slab surface temperature in the vicinity of the final solidified portion is strongly cooled to a range of 700 to 800 ° C. by increasing the amount of secondary cooling water.
The bulging generated between the rolls is suppressed by increasing the thickness of the solidified shell, and the slab is further rolled down with a light rolling roll at a strain rate of 0.2 to 0.4% per minute to form a concentrated molten steel. A method for inhibiting flow is disclosed.

However, under a light pressure slightly exceeding the amount of solidification shrinkage caused by the above-mentioned reduction rolls, the solidification shrinkage and bulging cannot be sufficiently prevented because only a point-wise reduction in the longitudinal direction of the slab is possible. . Further, since each reduction acts as a concentrated load, internal cracks are easily generated at the solidification interface, and there is a disadvantage that the amount of reduction cannot be increased.

Further, a method has been proposed in which forging is continuously performed near the solidification completion point in the center of the slab using a flat mold, but this method has the disadvantage that the processing equipment becomes large and the equipment cost increases. There is.

In order to solve the above-mentioned disadvantage, Japanese Patent Application Laid-Open No.
Japanese Patent No. 42460 discloses that dendrites are cut by molten steel flow using an electromagnetic stirrer or an ultrasonic application device installed on the upstream side near the solidification completion point of a slab to form an equiaxed crystal region near the solidification completion point. Then, a large reduction of 3 mm or more, which is larger than the solidification shrinkage, is given by a reduction roll arranged immediately before the solidification completion point of the slab, and the unsolidified molten steel is discharged to the upper molten portion to forcibly promote solidification. A method of forming a solidification completion point and eliminating center segregation without causing internal cracks is disclosed.

However, in this method, since the solidified portions at both ends of the slab having large deformation resistance are pressed down and plastically deformed, a steel type having large deformation resistance or a case where both ends of the slab have a low temperature and the deformation resistance is increased. However, there is a problem that a sufficient rolling effect cannot be obtained due to bending of the rolling roll and bending of the frame.

In order to solve the above problem, Japanese Patent Laid-Open Publication No.
No. 247 discloses a method in which an unsolidified portion at the center in the width direction of a slab is locally reduced by a stepped roll called a camel crown roll provided with a protruding portion at the center of a large-diameter roll. ing. However, in this method, a concave portion is formed on the surface of the slab due to the local roll-down by the stepped roll, which causes dimensional defects and flatness defects in the subsequent rolling process.

Further, due to the flow of the molten steel in the unsolidified portion in the slab and the variation of the secondary cooling, the unsolidified portion is not necessarily located at the central portion in the width direction near the point before the solidification end point of the slab. Does not coincide with the position of the camel roll protrusion, so that there is a drawback that the rolling down position cannot be properly maintained.

In order to solve this problem, the present inventor disclosed in Japanese Patent Application Laid-Open
Japanese Patent Application Publication No. -57410 proposes a method in which a slab including an unsolidified portion is once bulged, and the bulging amount is reduced immediately before the solidification completion point.

[0013]

However, even with the method described in Japanese Patent Application Laid-Open No. 9-57410, when a slab having insufficient surface cooling and high surface temperature is rolled down, internal cracks occur at the solidification interface. Or the improvement of center segregation cannot be obtained.
It turned out that there was a problem such as. The present invention relates to a method disclosed in JP-A-9-5.
No. 7410, which is a further improved method.

An object of the present invention is to solve the conventional problem that has arisen when a slab including an unsolidified portion is once bulged and the bulging amount corresponding to the bulging amount is reduced immediately before the solidification completion point. It is an object of the present invention to provide an advantageous continuous casting method for producing pieces.

[0015]

Means for Solving the Problems The present inventors have conducted studies for solving the above-mentioned problems and obtained the following findings.

(A) In a method of once bulging a slab and reducing the bulging equivalent immediately before the completion of solidification, when the slab surface temperature is reduced below a specific temperature, no internal cracks occur and center segregation occurs. Improves. (b) In the above method, it is necessary to perform the reduction at a position in the drawing direction where the unsolidified thickness after the reduction is equal to or less than a specific thickness.

(C) By controlling the specific water amount of the secondary cooling of the slab to a specific range and performing the above method, it is possible to optimize the casting speed. (d) When the slab having an unsolidified portion having an equiaxed crystal structure is reduced by the above method, the center segregation is more effectively improved.

The present invention is based on the above findings, and the gist is as follows (1) to (3).

(1) The slab including the unsolidified portion is once bulged, and after the bulging is formed, the unsolidified thickness after the reduction is equal to 1 m of the short side length of the mold.
A continuous casting method for steel, characterized in that the slab surface temperature before reduction is controlled to be less than 1000 ° C. and reduced at a position in the slab withdrawal direction of 0.0% or less.

(2) The specific water volume in the secondary cooling of the slab from immediately below the mold to immediately before the above-mentioned reduction is 1.0 l / kg · steel or more and 3.0 l
/ Kg · steel or less, the method for continuous casting of steel according to the above item (1), wherein

(3) The steel according to the above (1) and (2), wherein a treatment for generating an equiaxed crystal is performed in an unsolidified portion of the slab before performing a reduction equivalent to bulging. Continuous casting method.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic diagram of a longitudinal section showing an example of a device configuration of a continuous casting machine for carrying out the method of the present invention.

As shown in FIG. 1, the molten steel 8 injected into the mold 1 through the immersion nozzle 10 is cooled by spray water injected from the mold 1 and a nozzle (not shown) below the mold 1 and solidified. The shell 2a is formed into the slab 2, and the slab 2 is pulled out by the pinch roll 7 via the guide roll 3 and the pressing roll 5 while holding the unsolidified portion 2b inside. FIG. 1 shows an example of a vertical continuous casting machine, but the present invention can also be applied to a curved continuous casting machine. The electromagnetic stirring device 4 is a device for imparting stirring to the unsolidified portion 2b to make the structure of the unsolidified portion equiaxed as described later.

According to the method of the present invention, in the continuous casting machine having the above-described apparatus configuration, the guide roll 3 provided in the bulging zone shown in FIG. The slab 2 at the end of the bulging zone has a length of 20 mm to 100 mm.
Bulging to a degree, then roll 5 in roll-down zone
As a result, the pressure is reduced by the amount corresponding to the bulging amount. In addition,
Hereinafter, in the description of the present specification, only the short side of the slab is considered.

According to the present invention, after the bulging is formed, when the bulging equivalent amount is reduced, the unsolidified thickness after the reduction is 10.0% or less of the length of the short side of the mold in the drawing direction. And the surface temperature of the slab before rolling is 100
Control the temperature to less than 0 ° C and reduce the pressure.

Next, the surface temperature of the slab before rolling is set to 1000
The reason why the temperature is lower than ° C will be described.

FIG. 2 is an enlarged cross-sectional view of a reduction zone schematically showing the state of occurrence of internal cracks in the slab. FIG. 2A is a view when the surface temperature of the slab is less than 1000 ° C. Is 100
This is the case when the temperature is 0 ° C. or higher. In this figure, for the sake of convenience, an example of continuous casting in the horizontal direction will be described as an example. Is represented by V.

As shown in the figure, when the slab having the unsolidified portion 2b is reduced, a tensile stress acts on the solidification interface side of the solidified shell 2a on the long side, and the dendrites near the solid-liquid interface break. The surrounding concentrated molten steel is sucked between the dendrites.

If the surface temperature of the slab before rolling is less than 1000 ° C., the temperature gradient in the slab thickness direction becomes large, so that the rolling penetrates to the center in the slab thickness direction and the molten steel flow V Is promoted. Accordingly, as shown in FIG. 2A, the concentrated molten steel once sucked between the dendrites is discharged to the solidification front surface and does not become an internal crack.

On the other hand, the surface temperature of the slab is 1000 ° C.
In the above case, it is difficult for the reduction to penetrate to the center in the slab thickness direction. Therefore, the molten steel flow V is small, and FIG.
As shown in (1), the concentrated molten steel solidifies without being discharged, and the remaining portion of the concentrated molten steel becomes an internal crack.

Preferably, the lower limit of the surface temperature of the slab is 6
00 ° C. If the surface temperature is lower than this, the rolling force increases remarkably, and troubles such as inability to pull out the slab and breakage of the rolling roll are likely to occur.

Next, the reason why the reduction is performed at the position in the slab drawing direction where the unsolidified thickness after the reduction is 10.0% or less of the length of the short side of the mold will be described. Experiments have shown that even when the rolling is performed at the above-described surface temperature, the effect of improving the center segregation varies depending on the thickness α of the unsolidified portion after the rolling.

That is, when the unsolidified thickness α after the rolling is reduced at a position where it is larger than 10.0% of the length of the short side of the mold,
The discharge of the concentrated molten steel due to the reduction becomes insufficient, and center segregation occurs in a portion solidified after the reduction as in the conventional case, and the effect of the reduction cannot be sufficiently enjoyed. Here, the unsolidified thickness α after reduction
Is the thickness of the solid-liquid interface with a solid fraction of fs = 0.8, and is based on the temperature (T) distribution in the slab thickness direction calculated by one-dimensional unsteady heat transfer analysis in the slab thickness direction. You can ask.

The lower limit of the unsolidified thickness α after the reduction is 0, but a large reduction force is required if the reduction is performed so that the solidification completion point is forcibly formed. In particular, in the case of a large slab, a reduction device for the slab is required to be large enough that it cannot be industrially practically used. Therefore, for the purpose of reducing the diameter of the reduction roll and the size of the reduction device, it is preferable to perform the reduction to such an extent that the solidification completion point is not forcibly formed. Preferably, the unsolidified thickness after the reduction is 2.0% or more and 10.0% or less of the length of the short side of the mold.

In order to reduce the slab at an appropriate position and at an appropriate surface temperature, it is important to control the unsolidified thickness and the slab temperature. These two controls are to measure or calculate the solidified shell thickness and the slab surface temperature immediately before the reduction,
This is performed by adjusting the casting speed and the amount of secondary cooling water.

Next, as a preferred embodiment of the present invention, the specific water volume of the secondary cooling of the slab from immediately below the mold to immediately before the reduction is 1.0 liter.
The reason for controlling the pressure to be not less than / l / kg · steel and not more than 3.0 l / kg · steel will be described. Here, the specific water amount is defined as a cooling water amount per 1 kg of a slab.

As described above, the control of the unsolidified thickness and the slab surface temperature is performed by adjusting the casting speed and the amount of secondary cooling water. When the specific water volume of the slab secondary cooling is less than 1.0 l / kg · steel, a significant reduction in casting speed is required to secure an appropriate unsolidified thickness and slab surface temperature, and productivity is significantly reduced. . On the other hand, when the above specific water volume is larger than 3.0 l / kg · steel, the casting speed becomes excessive and breakout tends to occur.

Next, as a preferred embodiment of the present invention, the reason why a process for generating an equiaxed crystal in an unsolidified portion of a slab before rolling is described. The solidified structure at the center of the slab usually has a columnar crystal structure. In the case of the columnar crystal structure, bridging occurs in the width direction of the slab, and the effect of improving segregation tends to be small.
On the other hand, in the case of the equiaxed crystal structure, the molten steel flows easily due to the reduction and the local accumulation of the concentrated molten steel is prevented, so that the effect of improving the segregation is large.

Therefore, as shown in FIG. 1, the unsolidified portion 2 is formed by the electromagnetic stirrer 4 provided upstream of the reduction zone.
b is agitated to generate an equiaxed crystal in the unsolidified portion 2b at the center of the slab in the bulging zone, and thereafter, the above-described reduction is performed.

The electromagnetic stirring device 4 has a frequency of 1.0 to 3.
It is desirable to use a device having a frequency of 0 Hz and an applied current of about 600 to 900 A, and to set it in a bulging zone so as not to overlap with the rolling-down zone.

As a method for generating an equiaxed crystal,
It is not always necessary to use electromagnetic stirring. For example, a method of applying ultrasonic waves to the slab 2 via the guide roll 3 or the pressing roll 5 may be used. In addition, low-temperature casting and addition of a steel wire into a mold in consideration of simplicity and effects from the operation side can also be adopted as methods for generating equiaxed crystals.

[0042]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Using a continuous slab casting apparatus having the apparatus configuration shown in FIG. 1, a steel sheet of 40 kg class for a thick plate was produced under the six types of casting conditions A, B, C, D, E and F shown in Table 1. Manufactured. The bulging amount before the reduction was 20 mm, and the reduction was equivalent to the bulging with one reduction roll having a diameter of 350 mm. Inventive Examples A, B, C and D have a specific water content of 1.0 to 2.1.
1 / kg · steel, and the unsolidified thickness after reduction and the slab surface temperature were controlled to appropriate values. Comparative Examples E and F
Did not control the unsolidified thickness and the slab surface temperature. The electromagnetic stirrer was installed in a bulging zone, and by operating this, an equiaxed crystal was formed at the center in the slab thickness direction.

[0043]

[Table 1]

The state of occurrence of center segregation and internal cracking of the cast slab was examined. The center segregation was evaluated by the maximum segregation degree of [P] and the number of semi-macro segregated grains.

The maximum degree of segregation of [P] was determined by cutting the obtained slab at a cross section perpendicular to the casting direction, collecting a test piece from the center in the thickness direction, and cutting the surface of this sample at intervals of 200 μm. The mesh was divided into meshes, and the average concentration of [P] in each mesh was investigated. This [P] and the P concentration of the base molten steel [P0
] P / P0.

As for the number of segregated grains, a test piece was sampled from the center in the thickness direction of the slab in the same manner as described above, and the number of granular segregations in the range of 50 mm × 1000 mm was observed under a microscope of 50 times, and P / P 0 was 3
The above was investigated. Furthermore, the cross section of the slab was sulfur-printed to investigate the occurrence of internal cracks. Table 2 shows the maximum segregation degree of [P] and the occurrence of internal cracks.

[0047]

[Table 2]

As is clear from Table 2, Examples A to D of the present invention
In Comparative Example, the maximum degree of segregation was reduced, and no internal cracks occurred, as compared with Comparative Example. FIG. 3 is a graph showing the relationship between the number of segregated grains and the semi-macro segregated grain size.

As shown in the figure, the number of segregated grains in the example of the present invention was also reduced, and it was found that semi-macro segregation was significantly improved. Further, as shown in Tables 1, 2 and FIG. 3, it was found that there was a synergistic effect of improving center segregation by using electromagnetic stirring in combination.

[0050]

According to the method of the present invention, center segregation including semi-macro segregation can be improved without causing internal cracks.

[Brief description of the drawings]

FIG. 1 is a schematic view of a longitudinal section showing an example of an apparatus configuration of a continuous casting machine for carrying out a method of the present invention.

FIG. 2 is an enlarged cross-sectional view of a rolling zone schematically showing the state of occurrence of internal cracks in a slab. FIG.
In the case where the temperature is lower than 000 ° C., FIG.

FIG. 3 is a graph showing the relationship between the number of segregated grains and the semi-macro segregated grain size.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold 2 Cast piece 2a Solidified shell 2b Unsolidified part 3 Guide roll 4 Electromagnetic stirrer 5 Roller 6 Roller 7 Pinch roll 8 Molten steel 9 Crater end 10 Immersion nozzle 11 Casting direction 12 Internal crack α Unsolidified thickness after reduction V Flow of molten steel in unsolidified part

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-9-206903 (JP, A) JP-A-8-224650 (JP, A) JP-A 8-132203 (JP, A) JP-A-60-1985 21150 (JP, A) JP-A-60-6254 (JP, A) JP-A-61-42460 (JP, A) JP-A-61-132247 (JP, A) JP-A-63-252655 (JP, A) JP-A-9-57410 (JP, A) JP-A-11-33690 (JP, A) JP-A-9-314298 (JP, A) JP-A-2-235558 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/128 350 B22D 11/115 B22D 11/124 B22D 11/16

Claims (3)

(57) [Claims] 1. A slab including an unsolidified portion is bulged once, and after forming the bulging, when reducing the bulging equivalent, the unsolidified thickness after the reduction is 1% of the short side length of the mold.
A continuous casting method for steel, characterized in that the slab surface temperature before reduction is controlled to be less than 1000 ° C. and reduced at a position in the slab withdrawal direction of 0.0% or less. 2. The specific water volume of the slab secondary cooling from immediately below the mold to immediately before the reduction is set to 1.0 l / kg · steel or more and 3.0 l / kg.
The method for continuous casting of steel according to claim 1, wherein the steel is controlled to be not more than kg · steel. 3. The method for continuously casting steel according to claim 1, wherein a treatment for generating an equiaxed crystal is performed in an unsolidified portion of the cast slab before performing a reduction equivalent to bulging.