JPS6234458B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a continuous casting method for casting high-temperature defect-free slabs necessary for a process in which a hot rolling mill and a continuous casting machine are directly connected without going through a heating process in the steel manufacturing process. be.

In recent years, continuous casting machines (hereinafter referred to as continuous casting machines) have made remarkable progress in the steel industry, but on the other hand, demands on continuous casting machines are also extremely large. Among these, there is a remarkable demand for energy conservation due to the recent rise in energy prices. Particularly in the continuous casting-rolling process, there is a particularly great need for energy savings by directly connecting the continuous casting machine and rolling equipment.

In the steel industry, the conventional continuous casting-rolling process can be roughly divided into three processes shown in FIG. 13a, b, and c. The process shown in Figure 13a has been widely practiced in the steel industry for some time. Further, the hot charge process shown in FIG. 13b has been gradually put into practical use in the steel industry in recent years due to its remarkable energy saving properties. However, the direct rolling process shown in Figure 13c not only omits the heating step and requires no energy at all, but also eliminates the need to install heating equipment. Its development is greatly needed.

In the direct rolling process shown in FIG. It is an essential condition.

On the other hand, continuous casting machines can be roughly divided into vertical type, curved type,
There are curved types with vertical parts and horizontal types. The currently mainstream type and most commonly used is the curved continuous casting machine.

This curved continuous casting machine, as shown in FIG. 1, guides a curved slab 2 from a curved mold 1 into an arc shape with rolls 3, straightens it by straightening it with rolls 4, and performs casting.

In order to increase productivity (casting efficiency) with this curved continuous casting machine, it is necessary to increase the casting speed.
As the casting speed increases, it is inevitable that an unsolidified phase will exist inside the continuously cast slab during bend straightening.

In addition, in order to obtain high-temperature slabs, the conditions for secondary cooling by injecting water into the slab from directly below the mold to at least the bending straightening point are slow cooling, and the water injection is completed immediately after the bending straightening completion point, so that the It is necessary to solidify and reheat.

Furthermore, the machine height of a curved continuous casting machine is limited by the radius of curvature. When the machine height is high, the static pressure of molten steel increases. When the static pressure of molten steel increases, the amount of bulging of the slab increases, which causes internal cracking of the slab.

The arc radius R in this curved continuous casting machine is 3
There are various types within the range of ~13m, each with advantages and disadvantages. That is, if the arc radius R is increased,
As the machine height H increases, the amount of bulging between the rolls 3 and 3 increases due to the high static pressure of the molten steel, and when this bulging is corrected by the roll 3, tensile strain occurs at the solidification interface, resulting in internal cracks. occurs.
Therefore, it is necessary to increase the cooling intensity in the secondary cooling zone or to use split rolls to reduce the roll pitch. However, if the cooling strength is increased, internal cracks can be prevented, but unsolidified bending cannot be straightened, and a high-temperature slab that can be used in the direct rolling process cannot be obtained. On the other hand, theoretically it is possible to obtain high-temperature slabs without internal cracks by using multi-segmented rolls and reducing the roll pitch, but in reality there are problems with maintenance of the slab guide rolls.
It is impossible to stably obtain high-temperature defect-free slabs.

For example, conventional technical measures taken to prevent internal cracks in slabs obtained by continuous casting include supporting and guiding the slab after the mold, from the perspective of reducing bulging strain and correction strain. Measures have been taken to make the roll pitch denser to reduce the amount of bulging, thereby reducing bulging distortion. In addition, we aim to improve the hot strength of the solidified shell by using strong cooling (water injection ratio of 1.0/Kg or more) in the secondary cooling zone.

Currently, in most continuous casting machines using curved continuous casting machines, unsolidified bend straightening is performed, for example, arc radius R: 10 to 13 m, casting speed:
Continuous casting is performed under the following specifications: 0.7 to 2.0 m/min, dense roll arrangement, and spray cooling. When molten steel is cast using a curved continuous casting machine with these general specifications, the bending straightening point begins from the meniscus position.
It will be located between 15.7 and 20.4 meters. At this time, the surface temperature of the slab at the bending straightening point is 700 to 900℃,
The solidified shell thickness is approximately 80-120 mm (estimated). By the way, when the cross-sectional dimensions of the slab are 250 mm and the width is 1800 mm, 70 to 90% of the thickness of the slab is occupied by the solidified shell. If the slab is bent and straightened in this state, internal cracks will occur even under today's advanced technology.

On the other hand, when the arc radius R is made smaller, the machine height H becomes lower, the molten steel static pressure becomes smaller, the load on the rolls that support the slab is reduced, and the roll diameter becomes smaller.
Compared to a continuous casting machine with a large arc radius R and a high machine height, it is possible to make the roll pitch smaller, and the amount of bulging between the rolls can be reduced. Therefore, it is more suitable for obtaining high-temperature slabs through high-speed, slow cooling, unsolidified straightening, and reheating, when comprehensively judged by taking into consideration the maintainability of the slab support guide rolls, equipment costs, etc. be.

However, at the stage of immediately straightening the unsolidified slab, the tensile strain of the shell on the inside of the arc increases. That is, as shown in FIG. 2, it has an unsolidified phase 6,
When the bending is straightened during casting in the direction of the arrow 10, the shell 8 having a short length with respect to the neutral axis 7 of bending is generated due to pressure vibration deformation of the shell 9 having a long length, and a circular arc As the radius becomes smaller, this bending correction strain becomes larger.

There are two ways to alleviate this straightening strain: using multiple straightening points to disperse the straightening strain, and installing drive rolls in front of the straightening points (orthodontic band in the case of multi-point straightening) to create a solidified shell. Push in and
There is a method using so-called compression casting, in which the tensile strain generated in the upper shell is offset or alleviated by applying braking force by a group of drive rolls after the straightening point (or straightening zone).

However, in a multi-point straightening type curved continuous casting machine that uses multiple straightening points to disperse straightening strain, there is a limit to the length of the multi-point straightening band, and as a result, there is a limit to the number of straightening points. That is, in a multi-point straightening curved continuous casting machine, the straightening strain εn accompanying the n-th straightening is expressed by the following equation.

εn=D-S/2(1/Rn _- 1-1/Rn) In the above formula, D: slab thickness, S: shell thickness,
Rn _-1 ; Arc radius up to the nth correction point,
Rn: arc radius after the nth correction point.

Here, to simplify the explanation, the reference arc from the mold is
Consider performing one correction from R ₀ . In Figure 3, if we consider the case of bending back from R ₀ to R ₁ (>R ₀ ) at points A and B, point A is closer to the mold.
In other words, when the angle θ with the horizontal is smaller,
Machine height H after final correction becomes higher by ΔH. In actual cases, there will be multiple points (two or more), but the phenomenon is the same: the closer you start straightening to the mold, the higher the machine height becomes, making it more difficult to reduce the amount of bulging, resulting in bulging distortion. As is clear from the above distortion equation, there is also an influence of the shell thickness S, and the larger the shell thickness S, the smaller the corrective strain becomes. Therefore, from the viewpoint of preventing internal cracking by reducing bulging strain and correction strain, it is desirable to perform correction from a point where the angle θ is as large as possible.

On the other hand, as a matter of course, the angle θ is smaller than 90°, and therefore the length of the straightening band is almost determined once the reference arc R ₀ and the height H of the continuous casting machine are determined, and once the diameter of the roll that supports the static pressure is determined. The number of rolls that can be incorporated into the correction band is determined, and the number of correction points is also determined. In other words, as the arc radius R of a curved continuous casting machine becomes smaller, although the straightening strain increases, the number of straightening points cannot be increased as much. If there is, it will be about 15 points correction at most.

However, such a number of correction points can only be obtained through roll position control of about 0.1 mm, and can only be considered on design drawings. In actual cases, there is a deviation from the reference position of roll alignment, and even if the best technology is used and management is strict, a misalignment of 0.5 mm or less is unavoidable. Therefore, the number of effective straightening points is significantly reduced. For example, as described below, even though the standard arc radius is 3 m and the number of straightening points is 15, when the slab thickness is 250 mm, the casting speed is 1.5 m/min, and the water injection is ratio
Under casting conditions of 0.5/Kg, slabs without internal cracks could not be obtained. In addition, when the standard arc radius is made larger, the length of the straightening belt and the number of straightening points can be increased if the machine height is ignored, but as mentioned above, the degree of increase in the machine height is greater than when the standard arc radius is small. , the machine height increases, making it impossible to create a curved continuous casting machine with a low machine height.

By the way, according to the study results described later by the present inventors, when a direct rolling process, that is, direct rolling is enabled and high productivity is assumed, as described later,
The average temperature of the slab cross section at the end of the continuous casting machine is 1180
℃ or more, and based on the current roll support mechanism, it was found that it is desirable for the continuous casting machine to have a reference arc radius of 6 m or less and a machine height of approximately 6.5 m or less.

On the other hand, regarding the minimum machine height, if high-quality slabs are to be obtained using the current curved continuous slab casting machine, immersion-type powder casting is a prerequisite. The arc radius R ₀ must be 3.0 m or more for a 250 mm thick slab, and therefore the machine height H (R ₀ ) is 3.0 m.
m or more.

The above-mentioned compression casting also has the following problems when applied to a small arc, multi-point straightening, low machine height curved continuous casting machine (hereinafter abbreviated as low head continuous casting machine).

Figure 4 shows the profiles of a low-head continuous casting machine and a high-machine, high-curve continuous casting machine with a large arc single-point correction type (hereinafter referred to as the "high-head continuous casting machine"). B indicates an area in which a pushing drive roll can be placed, C indicates a horizontal brake zone, D indicates a multi-point correction zone, and E indicates a correction point. That is, in the case of a low-head continuous casting machine, as shown in Fig. 4, the drive generation band B of a single circular arc portion to generate the required compressive force in the straightening band D cannot be sufficiently secured, and the static pressure in the drive force generation area B is insufficient. but,
Since it is smaller than a high-head continuous casting machine, the driving force generated per driving force roll is small, and a sufficient correction strain relaxation effect cannot be obtained.

As shown in Figure 5, by compression casting (with CPC) with a standard arc radius of 5 mR and 15 points of correction, it was possible to reduce the strain at the rear stage of the orthodontic belt, but it was not possible to reduce the distortion at the front stage of the orthodontic belt. , slab thickness 250mm, casting speed 1.5
Compression casting was carried out under conditions of m/min, but it was not possible to completely eliminate internal cracks. The total strain on the vertical axis in FIG. 5 is the total strain of corrective strain, bulging strain, and misroll strain.

As mentioned above, it is not possible to obtain high-temperature slabs without internal cracks to achieve the direct rolling process, even when continuous casting is performed using a low-head continuous caster, and even when compression casting is performed using a low-head continuous caster. Moreover, as mentioned above, in high-head continuous casting machines, there is no internal cracking to realize the direct rolling process due to the distortion caused by bulging between the rolls, and high-temperature slabs cannot be obtained. , it is necessary to try to alleviate the correction distortion by new technical means.

The present invention promotes relaxation of tensile strain in the upper shell by adjusting the temperatures of the upper and lower shells of the slab and the shorter side shells during straightening, thereby obtaining a slab without internal cracks.

Conventionally, it has been proposed in Japanese Patent Application Laid-Open No. 52-52126 to increase the shell strength, reduce the amount of tensile strain, and prevent internal cracks due to bending straightening by lowering the temperature of the upper shell than that of the lower shell during straightening. This is a case of a high-head continuous casting machine with an arc radius of 10.5 m and a single point correction type, as shown in the examples of these publications, and will be described later. Like reference arc radius 3~6
m, using a multi-point straightening type low-head continuous casting machine with a machine height of 6.5 m or less, the arc inner shell surface temperature was set at 850℃,
It was not possible to prevent internal cracks by simply increasing the surface temperature of the outer shell to 1000°C. That is, in the case of a low-head continuous casting machine, it has been found that internal cracks occur only under the condition of a temperature difference between the inside of the arc and the outside shell.

The present invention connects the upper shell and the lower shell,
We analyzed the behavior of the short side, which transmits stress during straightening, and actively caused shear deformation of the short side shell by increasing the temperature of the short side shell higher than the temperature of the top shell. This was based on new knowledge that it can reduce the tensile strain of the upper shell.

The gist of the invention is as follows.

A multi-point straightening curved continuous casting machine with a machine height of 6.5m or less.
In a continuous casting method of bending and straightening a curved slab having an unsolidified phase at a high casting speed of 1.5 m/min or more, the side (inside) where tensile stress occurs during bending straightening of the slab having an unsolidified phase ( The following (1), (2), (3) are obtained between the surface temperature T _L of the upper (top) shell, the surface temperature T _F of the (bottom) shell on the side where compressive stress occurs (outside), and the surface temperature T _S of the short side shell. ) and (4), the bend straightening is started and completed, and the surface temperatures T _L and T _F at the point of completion of straightening are set to 800°C or higher, and non-solidified reheating is performed. Continuous casting method.

1000℃T _L 700℃ ……(1) T _F (=T _L +ΔT) 1100℃ ……(2) 200℃+1/4 (T _L −800℃) ΔT60℃+1/5 (T _L −800℃) ...(3) 1100°C + 2/3 (T _L -800°C) T _S 1000°C + 2/3 (T _L -800°C) ...(4) The continuous casting method of the present invention will be explained in detail below. First, the principle of relaxation of tensile strain in the top shell due to the temperature of the short side shell will be explained.

Although the short side 14 shown in Fig. 6 maintains the balance of stress on the upper and lower surfaces, it has traditionally been viewed as merely transmitting stress, and bending correction has been understood in one dimension. . However, as shown in FIG. 6, when looking at the bending straightening three-dimensionally, the casting is performed in the direction of the arrow 11, and when the straightening is performed at this point, tensile stresses shown at 15 and 15' act on the upper shell 12. Compressive stress indicated by 16 and 16' acts on the lower shell 13, and at this time, the upper shell undergoes deformation indicated by 17 and 17' to narrow the width of the upper shell 12, and the lower shell 1
A deformation indicated by 18, 18' will occur which attempts to widen the width of 3. In this case, it is the short sides that may inhibit this deformation. That is, on the short side, as shown in Fig. 7, as a result, 19,1
Shear deformation in the casting direction indicated by 9′ and 20,2
A shearing deformation in the width direction indicated by 0' will occur, but if the temperature of the short side shell is lower than that of the upper shell and the lower shell, the shell rigidity increases and this deformation is made less likely to occur. As a result, the tensile and compressive stresses shown at 15, 15' and 16, 16' in FIG. 6 become large, and cracks occur due to tensile strain based on the tensile stress on the upper surface side.

In the present invention, taking the above phenomenon into consideration, we take a cooling difference between the upper and lower surfaces, and raise the temperature of the short side shell more than that of the upper shell.
1 in FIG.
This makes it possible to reduce the tensile stress of 5 and 15' and obtain a high-temperature slab without internal cracks.

Because the short side is generally cooled from two directions (thickness direction and width direction) due to its shape, the temperature of the short side shell is generally lower and the rigidity is greater than that of the other upper and lower shells. This makes it difficult for shear deformation to occur.

In the present invention, the temperature of the short side is increased to promote shear deformation in a plane parallel to the wide surface of the slab, and various methods can be considered as methods for increasing the temperature of the short side. For example, installing a drain plate with a width slightly narrower than the width of the slab, implementing an air curtain,
If multiple nozzles are installed in the width direction,
Examples include a method of cutting off cooling water from a nozzle at the end of the slab depending on the width of the slab.

The present invention will be explained in more detail with reference to Examples.

In a continuous casting machine with a machine height of 3.5 m that performs 15 consecutive multi-point corrections in the section (4 m section) from 3 m below the meniscus to 7 m (4 m section) with a basic arc of 3 m,
A draining plate 21 as shown in FIG.
When installed at the 125 mm position and cut off the cooling water from the nozzle 23, and when the drain plate 21 is not installed,
Steel type: Medium coal Al-Si killed steel, slab size: 250mm
Casting was performed under the following conditions: thickness x 1500 mm width, casting speed: 1.5 m/min, and water injection ratio: 0.3 to 0.5/Kg, and the presence or absence of internal cracks was investigated.

Figures 9a and b show the results of the surface temperature of the upper surface (inner surface,
The figures are organized and illustrated according to surface temperature differences (external surface, L surface, F surface). Further, FIG. 10 shows the relationship between the surface temperature of the upper surface (inner surface, L surface) and the surface temperature of the short side of the slab on the entry side of the straightening zone, using the presence or absence of the drain plate as a parameter.

Now, the surface temperature of the upper surface (inner surface, L surface) of the slab on the side where the straightening belt is entered is T _L , the surface temperature of the lower surface (outer surface, lower surface) is T _F ,
The surface temperature difference between the top and bottom (inner, outer, L, F surfaces) is ΔT
(=T _F -T _L ), when a drain plate is installed, the surface temperature difference condition on the straightening zone entry side to obtain slabs without internal cracks is 700℃T _L 1100 from Figure 9.
℃, T _F In the range of 1100℃, it is as follows.

700℃T _L 1100℃ T _F 1100℃ ΔT=T _F −T _L ΔT60℃+1/5 (T _L −800℃) In addition, if no drain plate is installed, the following will occur.

700℃T _L 1100℃ T _F 1100℃ ΔT=T _F −T _L ΔT200℃+1/4 (T _L −800℃) The maximum value of the surface temperature T _F and T _L on the entrance side of the straightening band is the bulging between the rolls. This is determined from the viewpoint of preventing internal cracks from occurring due to the temperature difference, and the temperature on the side where the orthodontic belt enters must be approximately 1100°C or less.

In addition, the minimum value of the surface temperature T _F and T _L on the entry side of the straightening zone is the slab rolling temperature required in the rolling process in the direct rolling process, and the average temperature of the slab cross section at the end of the continuous casting machine determined by this rolling temperature. , the surface temperature of the slab at the straightening completion point (straightening zone exit side) determined from the cross-sectional average temperature at the end of the machine, the cooling conditions in the straightening zone (whether or not there is recuperation of the slab, and the amount of recuperation if recuperation is present) , the amount of cooling without recuperation), and as described later, at the required end temperature of 1180℃, for example, without recuperation of the straightening zone, the temperature at the entrance of the straightening zone will exceed (800℃ + straightening zone cooling temperature ℃) There is a need to.

On the other hand, as shown in Fig. 10, the short side surface temperature T _S at the straightening start point (straightening zone entry side) changes greatly depending on whether or not a drain plate is installed. The surface temperature T _L of the upper surface (inner surface, L surface) is 100 to 300° C. higher, whereas if it is not installed, it is about 100 to 250° C. lower. For details, refer to the upper surface of the slab on the side where the orthodontic belt enters (inner side,
L side) If the surface temperature is T _L and the short side surface temperature is T _S , then when a drain plate is installed, the distance between the surface temperature T _L and the surface temperature T _S is 700℃ T _L from Figure 10.
In the range of 1100°C and T _F 1100°C, the following relationship holds true.

1000℃ + 2/3 (T _L -800℃) T _S 1100℃ + 2/3 (T _L -800℃) T _Snio = 1000℃ + 2/3 (T _L -800℃) T _Snax = 1100℃ + 2/3 ( T _L -800℃) T _Snio T _S T _S max If no drain plate is installed, the following relationship will apply.

600℃+5/6 (T _L -800℃) T _S 700℃+5/6 (T _L -800℃) T _Snio = 600℃ + 2/6 (T _L -800℃) T _Snax = 700℃ + 5/6 ( T _L -800℃) T _Snio T _S T _S max From Figures 9 and 10, the short side shell is strongly cooled without installing a drain plate, and the temperature of the short side shell on the straightening band entry side is
The temperature of the upper surface (inner surface, L surface) will be lower than the surface temperature. In other words, the short side surface temperature T _S on the side where the orthodontic band enters
is the upper surface (inner surface, L surface) surface temperature T _L (700℃T _L
If the relationship becomes 600℃ + 5/6 (T _L -800℃) T _S 700℃ + 5/6 (T _L -800℃) with respect to 1100℃), a defect-free slab without internal cracks will be obtained. Upper and lower surfaces (inner, outer, L, F) on the side where the orthodontic band is inserted
The temperature difference condition for the upper surface (inner surface, L surface) is ΔT200℃+1/4 (T _L −800℃), while the upper surface (inner surface, L surface) surface temperature T _L
is 700℃T _L 1100℃, the bottom surface (outer surface, F surface) surface temperature T _F is constrained by T _F 1100℃, and ΔT=
Since T _F - T _L , the upper surface (inner surface, L surface) surface temperature T _L that satisfies the above temperature difference condition and also satisfies the upper surface, lower surface (inner surface, outer surface, L, F surface) temperature constraints is 700℃.
T _L is in the range of 880℃.

That is, in order to obtain a defect-free slab without internal cracks, the upper and lower surfaces (inner, outer surfaces, L, F
The temperature difference conditions for the upper surface (inner surface, L surface) and the upper surface (inner surface, L surface) are 1100℃ - _{T L} ΔT200℃ + 1/4 (T _L - 800℃), 700℃T _L 880℃, as shown in Figure 9b. It is limited to a narrow condition range surrounded by points A-B-C.

On the other hand, as shown in Figures 9 and 10, a drain plate is installed to slowly cool the short side shell and raise the temperature of the short side shell on the side where the straightening belt is entered to be higher than the upper surface (inner surface, L surface) shell temperature. On the obi side, the short side surface temperature T
_S , upper surface (inner surface, L surface) surface temperature T _L (700℃
When maintaining the relationship of 1000℃ + 2/3 (T _L -800℃) T _S 1100℃ + 2/3 (T _L -800℃) with respect to T L <1100℃), defect-free slabs with no internal cracks can be _obtained . The upper and lower surfaces (inner, outer, L, F
The temperature difference condition is ΔT60°C + 1/5 (T _L -800°C), while the upper surface (inner surface, L surface) surface temperature T
_L is constrained by 700℃T _L 1100℃, the bottom surface (outer surface, F surface) surface temperature T _F is constrained by T _F 1100℃, and ΔT=
Since T _F -T _L , the upper surface (inner surface, L surface) surface temperature T _L that satisfies the above temperature difference condition and each surface temperature condition is in the range of 700°C T _L 1000°C.

That is, in order to obtain a defect-free slab without internal cracks, the above temperature difference conditions and upper surface temperature conditions at the entrance side of the straightening zone are 1100°C - _{T L} ΔT60°C + 1/5 (T _L - 800°C), 700°C T _L becomes 1000°C, which is expanded to a wide range of conditions surrounded by points A-DE-E shown in Figure 9b. The condition range surrounded by points B-D-E-C in Figure 9b [1000℃≧T _L ≧700℃, T _F (=T _L +Δ
T)≦1100℃, 200℃＋1/4 (T _L -800℃)≧ΔT≧6
0 °C + 1/5 (T _L -800 °C), 1100 °C + 2/3 (T _L -800
℃) ≧ T _S ≧ 1000℃ + 2/3 (T _L -800℃)] is the enlarged part, and the enlarged part surrounded by B-C-F-D is 880
℃≧T _L ≧700℃, 200℃＋1/4 (T _L −800℃) ≧ΔT ≧60℃＋1/5 (T _L −800℃), 1100℃＋2/3 (T _L
-800°C)≧T _S ≧1000°C + 2/3 (T _L -800°C).

Furthermore, from Figures 9 and 10, a drain plate is installed and the short side shell is slowly cooled, and the temperature of the short side shell is adjusted to the upper surface (inner surface, L).
In other words, raise the short side surface temperature T _S on the side where the orthodontic band is inserted into the upper surface (inner surface, L surface).
For surface temperature T _L (700℃T _L 1100℃),
1000℃+2/3 (T _L -800℃) T _S 1100℃+2/3
By maintaining the relationship (T _L −800° C.), the short side shell is strongly cooled without installing a drain plate, and the short side shell temperature is lowered below the upper surface (inner surface, L surface) surface temperature. In other words, the short side surface temperature T _S on the side where the orthodontic band enters
is 600 for the upper surface (inner surface, L surface) surface temperature T _L
℃5/6 (T _L -800℃) T _S 700℃ + 5/6 (T _L -8
00 °C), the minimum required upper part on the straightening band entrance side to prevent internal cracking.
Lower surface (inner, outer, L, F surface) temperature difference ΔT (=T _F
−T _L ) in the range of T _L = 700 to 880°C, Δ
T=200℃+1/4 (T _L -800℃) to ΔT=60℃+1
/5 (T _L -800℃).

For example, if the minimum value of the upper surface (inner surface, L surface) surface temperature T _L is 700°C under the implementation conditions in which slabs without internal cracks could be obtained without installing a drain plate, Δ
T can be reduced from 175°C to 40°C, and from 220°C to 76°C at the maximum value of 880°C. Also, at temperature T _L = 800℃,
ΔT can be reduced from 200℃ to 60℃. This shows that by maintaining the short side shell temperature higher than the top shell temperature on the straightening band entry side, the tensile strain occurring in the top shell can be effectively alleviated.

As mentioned above, slowly cool the short side shell using a drain plate,
By making the short side shell temperature higher than the top shell temperature on the side where the straightening belt enters, it is possible to reduce the temperature difference between the top surface (inner surface, L surface) and the bottom surface (outer surface, F surface), which improves operational efficiency. It is possible to absorb the variation in the number of slabs and stably ensure defect-free slabs. For example, if a drain plate is not installed, the maximum surface temperature of the upper surface (inner surface, L surface) that can be obtained to obtain a high-temperature slab is 880°C.
Internal cracking cannot be prevented unless the bottom surface temperature T _F = 1100°C and temperature difference ΔT = 220°C are maintained. For example, if T _F exceeds 1100°C during operation, or T _L exceeds 880°C, the temperature difference ΔT If the temperature drops below 220℃, internal cracks will occur.

On the other hand, when a drain plate is installed, if T _L = 880℃ constant, T _F is 1100 to 965℃ (ΔT = 220 to 76℃)
Defect-free slabs with no internal cracks can be obtained even if variations occur within this range. For example, T _L is 700 to 900
Variations occur within the range of ℃ (or 800 to 900℃),
Moreover, even if ΔT varies within the range of 80 to 175°C (or 60 to 200°C), a defect-free slab can be reliably obtained.

Furthermore, as described above, by slowly cooling the short side shell using the drain plate and raising the temperature of the short side shell on the side where the straightening band is entered to be higher than the temperature of the upper surface shell, the upper surface (inner surface, inner surface,
It is possible to reduce the temperature difference between the L surface) and the lower surface (outer surface, F surface), absorb operational variations as mentioned above, and stably obtain defect-free slabs. First, the surface temperature of the upper surface (inner surface, L surface) of the shell can be made higher, and as a result, it is possible to obtain a defect-free slab at a higher temperature, and the rolling process side in the direct rolling process can be improved. Conditions will be relaxed.

That is, as mentioned above, the lower surface (outer surface,
F side) surface temperature T _F needs to be approximately 1100°C or less from the viewpoint of preventing internal cracks due to inter-roll bulging, and if there is no drain plate, the top surface (inner surface, L side) surface temperature T _L The maximum temperature is approximately 880℃, but if a drain plate is installed, the temperature can be increased to approximately 1000℃.

Specifically, when a drain plate is installed, it is impossible to obtain a defect-free slab without a drain plate, and the high temperature condition range that is preferable for obtaining a high-temperature slab is shown in Fig. 9. Temperature condition range surrounded by points C-FE-E shown in b [880℃T _L 1000℃, 1100℃
-T _L ΔT60°C + 1/5 (T _L −800°C), T _F (=T _L +ΔT) 1100°C], a defect-free slab can be obtained.

The above describes the casting results of a small arc with a standard arc of 3 m, machine height of 3.5 m, straightening band length of 4 m, multi-point straightening type low machine height, and curved continuous casting machine, and the slab surface temperature conditions on the straightening band entry side. The relationship between the occurrence of internal cracks and the occurrence of internal cracks has been organized and explained. Depending on the cooling conditions within the straightening zone, the top, bottom, and short side surface temperatures of the slab on the side where the straightening zone enters the straightening zone. It also changes on the side where the orthodontic band comes out.

For example, depending on the cooling conditions within the orthodontic zone, the temperature of each surface on the entrance side of the orthodontic zone may be maintained within the orthodontic zone, and the temperature of each surface on the exit side of the orthodontic zone may be maintained at the above-mentioned temperature on the entrance side. The surface temperature is gradually lowered, and the temperature of each surface on the belt exit side is slightly lower than the temperature of each surface on the belt input side,
By regenerating the heat within the straightening zone, the temperature of each surface on the belt exit side can be made higher than the temperature of each surface on the belt entry side.

However, if the cooling conditions are adjusted within the straightening zone and reheating is performed within the straightening zone, if the upper surface is within the straightening zone and the surface temperature is 100℃ or higher, the tensile strain due to this reheating will occur. It was found that internal cracks occur due to the addition of Therefore, the reheating within the straightening zone must be kept at 100° C. or lower, and the reheating step to obtain a high-temperature slab for the direct rolling process is preferably carried out at a time when the total strain is low after straightening is completed.

Furthermore, for cooling within the straightening zone, it is preferable to employ a flat cooling pattern in which the temperature at the entrance of the straightening zone is maintained or a slope cooling pattern in which the temperature is gradually lowered within the straightening zone.

Figures 9 and 10 are examples of a reference arc of 3 m.
In the case of a standard arc of 6 m, although there are advantages in reducing orthodontic strain due to an increase in the arc radius and in reducing orthodontic strain with a high shell thickness, there is an increase in the roll diameter due to an increase in static pressure, and bulging distortion due to an increase in roll pitch. As a result, the temperature conditions at the entrance of the straightening band are almost the same as in Figures 9 and 10, and the same phenomenon occurs when the casting speed increases. It was also found that almost the same conditions need to be met.

Next, the temperature conditions on the straightening band exit side (straightening completion point) will be described. The lower limit temperature condition at the time of completion of straightening is
It is determined by the temperature conditions of direct rolling. FIG. 11 shows an example of a cooling pattern in a low-height continuous casting machine in a direct rolling process. In this cooling pattern example, the flat cooling pattern described above is adopted as the cooling pattern within the straightening zone, and unsolidified heat is regenerated in the horizontal portion after the straightening is completed.

The problem here is the occurrence of internal cracks due to unsolidified recuperation in the horizontal section, and as a result of investigating this limit value, it was found that internal cracks will not occur if the amount of recuperation at the surface temperature is 260°C or less. I understand. Therefore, the average temperature of the slab cross section during complete solidification at a casting speed of 1.7 m/min and reheating conditions of 260°C or less in the horizontal section is calculated for each surface temperature at the point of completion of straightening and for each reference arc (R ₀ ). It will look like the figure.

Although there are slight differences depending on the steel type and rolling mill capacity, the average temperature of the slab cross section at the end of the continuous caster calculated from the rolling temperature is 1180℃, and as is clear from Fig. 12, the surface temperature It can be seen that it is necessary to complete the straightening at a temperature of 800°C or higher.

Therefore, in order to obtain a high-temperature slab for the direct rolling process (average slab cross-sectional temperature at the end of the continuous caster of 1180°C or higher), the surface temperature of the upper and lower surfaces of the slab at the straightening strip exit side (straightening completion point) must be 800°C. Must be above ℃.

Also, the relationship in Figure 12 is that the casting speed is 1.7 m/
min, but as long as the recuperation process is used, the temperature at the end of the continuous caster is dominated by the difference between the fully solidified position and the end of the casting machine, and the casting speed is 1.5 min.
It was found that within the range of m/min to 2.0 m/min, it hardly depends on the casting speed.

The gist of the present invention detailed above is summarized as follows. In order to obtain high-temperature slabs of good quality without internal cracks for direct rolling, we use a low machine height continuous casting machine with less bulging, i.e. a low machine height multi-point straightening curved casting machine. During casting with a continuous casting machine, more specifically, when bending and straightening a curved slab with an unsolidified phase, in order to alleviate the tensile strain caused by straightening, the shell surface temperature on the side where tensile stress occurs during straightening is adjusted to the side where compressive stress occurs. The surface temperature of the short side shell is lower than the surface temperature of the shell on the side where the tensile stress occurs, and the surface temperature of the short side shell is higher than the surface temperature of the shell on the side where the tensile stress occurs. By starting the bending straightening and completing the bending straightening, shear deformation is preferentially caused on the short side, and at least after the straightening is completed, unsolidified reheating is performed in order to obtain a high-temperature slab. This is a continuous casting method.

The continuous casting method of the present invention can also be carried out using a curved continuous casting machine having a vertical section.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of the profile of a continuously cast slab being cast by a curved continuous casting machine, Fig. 2 is an explanatory diagram of the principle that tensile strain occurs in the upper shell due to bending straightening,
Figure 3 is a schematic diagram showing the influence of the straightening zone start position on the machine height, Figure 4 is an explanatory diagram showing problems when performing compression casting in a low machine height type continuous casting machine, and Figure 5 is a schematic diagram showing the influence of the straightening band start position on the machine height. An explanatory diagram showing the degree of strain reduction when compression casting is performed with a high-type continuous casting machine, Fig. 6 is an explanatory diagram of the principal stress and principal strain when bending straightening is viewed three-dimensionally, and Fig. 7 is An explanatory diagram of the shear stress component generated in the short side shell. Figure 8 is an explanatory diagram of the installation situation of the drain plate installed to increase the temperature of the short side shell. Figures 9 a and b are the upper and lower surfaces of the straightening zone. An explanatory diagram of the influence of surface temperature and the presence or absence of a drain plate on the occurrence of internal cracks. Figure 10 is an explanatory diagram of the relationship between the top surface temperature and short side surface temperature with and without a drain plate installed. Figure 11 is a high-temperature defect-free diagram. A schematic diagram of a recuperative cooling pattern for manufacturing slabs; Figure 12 is an explanatory diagram of the relationship between the straightening completion temperature and the average cross-sectional temperature of the end slab; Figure 13;
Figures a, b, and c are diagrams showing the conventional continuous casting to rolling process. DESCRIPTION OF SYMBOLS 1... Curved mold, 2... Curved slab, 3, 4... Roll, 6... Unsolidified phase, 7... Neutral axis of bending, 8... Top shell, 9... Bottom shell, 10... Casting direction (arrow), 11... Casting direction (arrow), 12...Top shell, 13...Bottom shell, 14...Short side (shell),
15, 15'... tensile stress, 16, 16'... compressive stress, 17, 17'... deformation that attempts to narrow the width of the upper shell, 18, 18'... deformation that attempts to widen the width of the lower shell, 19, 19'... Shearing deformation in the casting direction, 20, 20'... Shearing deformation in the width direction, 21...
Drain plate, 22...roll, 23...nozzle.

Claims (1)

[Claims] 1. In a continuous casting method for bending and straightening a curved slab having an unsolidified phase at a high casting speed of 1.5 m/min or more using a multi-point straightening curved continuous casting machine with a machine height of 6.5 m or less. , the surface temperature T _L of the (top) shell on the side (inside) where tensile stress occurs during bend straightening of a slab having an unsolidified phase, the surface temperature T _F of the (bottom) shell on the side (outside) where compressive stress occurs, Bending correction is started while maintaining the relationships of the following equations (1), (2), (3), and (4) between the surface temperature T _S of the short side shell, the bending correction is completed, and the correction completion point is A continuous casting method characterized by setting the surface temperatures T _L and T _F at 800°C or higher and performing unsolidified reheating. 1000℃≧T _L ≧700℃ ……(1) T _F (=T _L +ΔT) ≦1100℃ ……(2) 200℃＋1/4 (T _L −800℃) ≧ΔT ≧60℃＋1/5 ( T _L −800℃) ……(3) 1100℃＋2/3(T _L −800℃)≧T _S ≧1000℃＋2/3(T _L −800℃) ……(4)