JP3065484B2 - Manufacturing method of thin cast slab by belt type 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 producing a thin slab by a belt type continuous casting method, and more particularly to a method for producing a thin slab having no surface cracks and internal cracks.

[0002]

2. Description of the Related Art Conventionally, cast slabs have been manufactured by continuously injecting molten steel into a water-cooled mold and continuously drawing the molten steel while cooling and solidifying it in the mold. This slab typically has a thickness of 15
It has a thickness of 0 to 300 mm, and in order to further reduce the thickness to a material having a desired thickness, it is necessary to obtain a hot working step that consumes extremely large energy. Therefore, it is desired to realize a further reduction in energy consumption as a real problem in an environment like today.

A belt-type continuous casting method developed to satisfy such a demand is disclosed in, for example, Japanese Patent Application Laid-Open No. 50-61332.
As shown in the publication, an endless metal belt circulating and moving by a driving roll is disposed opposite to both ends of the long side, and a block circulating in synchronization with the endless metal belt on both sides of the short side. A method in which these belts and blocks are circulated at a predetermined speed, and molten steel is continuously supplied to a space surrounded by the belts and blocks, and a thin slab is continuously drawn while being solidified by cooling. It is.

Although there are various proposals for a specific method of manufacturing the thin slab, the thickness of the slab that can be manufactured is approximately 100 mm or less, and by further reducing the thickness, the hot rough rolling can be omitted. And very large energy savings can be achieved.

On the other hand, in the belt type continuous casting method,
Since the slab size is small as described above, it is necessary to perform high-speed casting to achieve high productivity. However, when high-speed casting is performed, the cooling rate is too high and the cooling tends to be locally uneven locally. As a result, the solidification unevenness and the amount of strain increase, and vertical cracks occur on the slab surface. There is a problem that it is easy. Therefore, it is generally intended to slowly cool the slab.
Japanese Patent Application Laid-Open Publication No. H11-157, discloses a belt-type continuous casting method for thin cast pieces, in which an endless metal belt surface is coated with a coating material such as alumina and the whole is slowly cooled to form a solidified shell having a uniform thickness. .

The present inventors have also disclosed in Japanese Patent Application No. Hei 5-3384.
In No. 18, a cooling index indicating the degree of slow cooling required for preventing vertical cracks is presented, and its value is determined according to the steel type component and the frictional force of the belt coating used.

[0007]

However, if slow cooling is performed, longitudinal cracks on the surface are reduced. However, since the thickness of the slab in the early stage of solidification is reduced, lateral cracks are generated and the slab is not rolled between rolls. There is a problem that cracking inside the slab due to bulging (bulging) and fluctuation in the level of the molten metal level in the mold become remarkable. Therefore, it is necessary to find casting conditions that do not cause vertical cracks, and that do not cause horizontal cracks, cracks inside the slab, and fluctuations in the mold level in the mold.

According to the present invention, by defining a melting point and a thickness of a coating material to be used, vertical cracks are not generated by using a cooling index derived from a certain relational expression from a dendrite secondary arm interval of a solidified structure, In addition, cooling conditions were determined to prevent the occurrence of lateral cracking, the occurrence of cracks inside the slab due to the slab swelling (bulging) between the rolls, and the fluctuation of the level of the molten metal in the mold. It is an object of the present invention to provide a method for producing a thin cast piece.

[0009]

In order to achieve the above object, the present invention has the following constitution. That is, on both sides of the long side, a circulating endless metal belt is arranged oppositely, and on both sides of the short side, a block that circulates and moves in synchronization with the endless metal belt is opposed to a belt-type continuous casting facility. In a method for producing thin cast slabs by supplying molten steel having a carbon concentration of 0.08% or more and less than 0.25%, one or more coating materials are melted with the melting point of the outermost coating material. 950 ° C. or less, the endless metal coated so that the thickness thereof is 60 μm or more and the cooling index R at a depth of 0.2 to 0.7 mm from the surface layer of the obtained slab is 60 to 160. This is a method for producing a thin cast piece by a belt-type continuous casting method, wherein casting is performed using a belt. R = (d / 191) ^-2.237 where d: average value of dendrite secondary arm spacing (μ
m)

[0010]

The present inventors have previously shown in Japanese Patent Application No. 5-338418 a cooling index indicating the magnitude of cooling strength of a slab. This expresses the cooling index R by the following conversion formula based on the dendrite secondary arm interval of the solidified structure of the slab cross section. R = (d / 191) ^-2.237 where d: average value of dendrite secondary arm spacing (μ
m)

According to this, in a steel having a carbon concentration of 0.08% or more and less than 0.25%, a cooling index at which no vertical crack occurs depends on the frictional force of the belt coating, for example, a friction coefficient of 0.1 or less. Defines that the cooling index R is 5 or more and 70 or less. However, it has been found that under these conditions, vertical cracks on the surface of the slab can be completely prevented, but horizontal cracks and cracks inside the slab tend to occur. This tendency is particularly remarkable in high speed casting. As a result of the solidification structure analysis, in such a case, the initial solidification shell thickness becomes thinner, and it becomes susceptible to slight deformation of the belt. Then, it has been found that so-called bulging occurs, in which the slab expands between the rolls due to the molten steel static pressure, and the shell is deformed, so that cracks are easily generated inside the slab. Furthermore, when bulging occurs,
The problem that the change in the level of the molten metal in the mold becomes remarkable also tends to occur.

Therefore, it was considered that the cooling rate of the slab should be increased to some extent. However, even if the cooling rate was increased, a device for preventing vertical cracks from occurring was required. Therefore, the inventors have paid attention to the fact that the range of the cooling index in which the vertical crack does not occur varies depending on the friction coefficient of the belt coating material, and conducted an experiment for changing the frictional force of the belt coating material. The coating material with the lowest coefficient of friction of the material itself is the oxide in a molten state. Therefore, it is premised to use a so-called molten coating material which can adjust the composition of the coating material so as to have a low melting point and can be melted when brought into contact with molten steel. However, it is considered impossible to further reduce the coefficient of friction of the molten oxide.

Therefore, the inventors focused on the thickness of the molten coating material and thought that the apparent frictional force would be reduced if the thickness exceeded a certain value. This is because, when the thickness is thin, the probability that the molten oxide liquid layer between the belt and the slab is partially broken increases, and when viewed as a whole surface,
This is because the apparent frictional force becomes larger than the frictional force determined from the actual friction coefficient. The area where the liquid layer is broken
Since it is no longer liquid lubrication, the frictional force becomes very large.

In order to change the apparent frictional force, a ZrO ₂ -based coating is applied between the belt and the outermost layer coating material in order to change the thickness of the outermost layer coating material and further change the cooling rate. A casting test was performed with a belt continuous caster while changing the thickness, and the results shown in FIG. 1 were obtained.

The cooling index shown here is obtained as follows. First, the cast slab is cut along a plane including a cross section perpendicular to the casting direction and also perpendicular to the slab thickness direction. This surface is mirror-polished and then picric acid (30 g / 500 g).
(cc + 10 cc surfactant) at 60 ° C for 5 minutes to reveal a coagulated structure. From the photograph magnified 10 times, the interval between secondary dendrite arms observed in the solidified structure was measured with a 10-magnifying loupe. From the surface layer.
The measurement was performed at a pitch of 50 mm in the width direction of the slab in a range of 0.2 mm to 0.7 mm centering on 5 mm. These average values were converted into a cooling index using the following relational expression. R = (d / 191) ^-2.237 where d: average value of dendrite secondary arm spacing (μ
m)

FIG. 1 shows that, even when a coating material having the same composition, which is in a molten state when brought into contact with molten steel, is used, the critical cooling index for preventing the occurrence of longitudinal cracks differs by changing the thickness. . From the figure, for example, when the thickness of the outermost layer of the molten coating material is 60 μm, the maximum cooling index at which no vertical crack occurs is 160.

Therefore, as originally expected, by increasing the thickness of the coating material to be melted on the outermost layer, the apparent frictional force does not increase, so that the upper limit cooling index at which no longitudinal cracking occurs is increased. It became possible.

On the other hand, in the belt casting machine, as a result of obtaining a cooling index for preventing the occurrence of lateral cracks, as shown in FIG. 2, when the cooling index became 60 or more, the occurrence of lateral cracks became zero. . In addition, FIG. 3 shows a cooling index which does not cause cracks in the slab due to bulging, and it is found that a cooling index of 30 or more is required. Further, FIG. 3 also shows a change in the level of the molten metal level in the mold due to bulging.
It can be seen that when it is 0 or more, it falls within the allowable range of operation ± 15 mm.

As the frictional force decreases, the upper limit of the cooling index required to prevent the occurrence of vertical cracks may be increased, that is, the reason why cooling may not be so required is as follows. Longitudinal cracking generally occurs when strain concentrates and accumulates in a portion where the initial solidification shell thickness when molten steel solidifies is delayed for some reason and exceeds the critical strain of cracking. At this time, there are two major causes of the distortion. One is caused by the thermal contraction caused by the temperature drop being hindered by the frictional force between the belt and the slab, and the other is caused by the δ / γ transformation. Occurs in occurrence. Slow cooling leads to prevention of longitudinal cracks due to the action of making the initial solidified shell thickness uniform and the action of relaxing strain accumulated in the delayed solidification portion. On the other hand, a decrease in frictional force suppresses the generation of distortion due to the inhibition of heat shrinkage of the slab. Therefore, if the frictional force can be reduced as much as possible, the amount of accumulated strain becomes small, so that cracking does not occur even if the strain relaxation effect by slow cooling is not so large.

Next, the reason for defining the conditions of the invention will be described. The target steel type was steel having a carbon concentration of 0.08% or more and less than 0.25%. This is the C
Is included, the amount of strain generated by the δ / γ transformation during solidification is large, and the thinned portion of the solidified shell is separated from the mold, which further promotes the solidification delay and easily causes cracking.

The reason why the upper limit of the cooling index is specified is that the maximum value of the cooling index for preventing the occurrence of vertical cracks is 160 or less as described above. The reason for defining the lower limit is that the lower limit value 60 at which lateral cracks do not occur, the lower limit value 30 of the cooling index for preventing internal cracks, and the lower limit value 40 for further reducing the fluctuation amount of the molten steel surface are set to the larger values. 60.

The reason why the thickness of the outermost layer coating material is specified to be 60 μm or more is as follows. In other words, in order to make the cooling index for preventing vertical cracks as large as possible, it is necessary to reduce the frictional force between the belt coating material and the slab as much as possible. Is the minimum thickness for ensuring a sufficient thickness.

The melting point of the outermost layer coating material must be as low as possible because the coating material applied to the belt is melted immediately before coming into contact with the molten steel in order to reduce frictional force as much as possible. Since the belt is water-cooled from the back side, it does not melt even if it has a melting point slightly lower than the molten steel temperature.
Therefore, 950 ° C. was set as the upper limit obtained from the experiment.

In the present invention, the cooling index R defined in this manner is determined in advance for each casting condition performed on the same steel type, and the casting condition of a newly manufactured slab is applied to this. When the cooling index is within the range of the cooling index of the present invention, it is possible to obtain a slab which does not cause vertical cracks.

As means for obtaining such a cooling index,
The thermal conductivity of the coating material applied to the belt (particularly, the coating material serving as a base) is reduced and the thickness is increased. Coating material is applied in multiple layers to increase the thermal resistance at the interface. Although there are methods such as increasing the casting speed, any method may be used in the present invention.

Further, it is difficult to measure the friction coefficient of the oxide in the molten state to be the outermost layer coating, so that it cannot be specified. However, the outermost layer coating conditions of the present invention, that is, the melting point is 95%
If the thickness can be maintained at 0 ° C. or less and the thickness can be maintained at 60 μm or more, longitudinal cracks will not occur, and it is considered that the condition is that the frictional force is sufficiently reduced.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In a continuous casting method using a mold composed of a pair of movable short sides composed of a pair of belts and a plurality of blocks that move in synchronization with the speed of a slab, the basic component ranges shown in Table 1 are shown. Was investigated, and the relationship between the cooling index obtained from the solidification structure, the occurrence of vertical cracks, the lateral cracks, and the internal cracks of the slab was investigated. Furthermore, the fluctuation of the molten steel level in the mold was investigated. At this time, in order to change the frictional force between the belt and the slab, the outermost coating material was of a so-called melting type in which the coating material was brought into contact with molten steel and melted, and the thickness was changed. Further, for the purpose of changing the cooling index, the coating material of the first layer was made of a ZrO ₂ -based oxide, and its thickness was changed.

The cast slab is cut along a plane including a cross section perpendicular to the casting direction and also perpendicular to the slab thickness direction. After mirror-polishing this surface, picric acid (30 g / 500 cc + 10 cc surfactant) is used. Etching was performed at 60 ° C. for 5 minutes to reveal a solidified structure. From the photo magnified 10 times,
The secondary dendrite arm spacing found in the coagulated tissue was measured with a 10x loupe. The measurement was performed at a pitch of 50 mm in the slab width direction in a range of 0.2 mm to 0.7 mm centered on 0.5 mm from the surface layer. These average values were converted into a cooling index using a relational expression.

[0029]

[Table 1]

[Table 2] Casting conditions Casting speed: 5 m / min to 10 m / min Slab size: 1300 mm width x 50 mm thickness, 1300 mm width x
75 mm thickness Belt coating conditions ・ Base coating: ZrO ₂ : Thickness 20, 50, 80, 100 μm ・ Top layer coating: Al ₂ O ₃ —SiO ₂ —CaO
—Na ₂ O type: melting point 910 ° C., 970 ° C .: thickness 10, 20, 40, 60, 80 μm

In order to obtain the condition of strong cooling, the cooling water amount on the back side of the belt was doubled in some levels.

First, the vertical cracks on the surface of the slab were investigated, and the relationship between the index based on the total length of the vertical cracks and the cooling index is shown in FIG.
The vertical cracking index has a large value when the cooling index is large, but the cooling rate at which no cracking occurs depends on the thickness of the outermost layer coating.
It is 60, and there is no change even if the thickness is further increased. In the case of the coating material B having a melting point of 970 ° C., the thickness is 8
Even if it is 0 μm, the vertical cracking does not decrease. This is probably because the belt did not melt sufficiently before contacting the molten steel.

FIG. 2 shows that the base coat is made of ZrO ₂ and a coating material A having a melting point of 910 ° C.
This shows the occurrence of lateral cracking when m coating was performed. The cooling index is changed by changing the thickness of the base coat. As shown in the figure, the occurrence of lateral cracks decreases as the cooling index increases, and the cooling index 60
And became zero.

FIG. 3 shows the relationship between cracks inside the slab due to bulging and the cooling index. When the cooling index is 30 or more, no internal cracks occur. Further, FIG. 3 also shows a change in the level of the molten metal level in the mold due to bulging, and it can be seen that when the cooling strength is 40 or more, the operation falls within the allowable range of ± 15 mm.

[0035]

As described above, according to the present invention, by setting a cooling index focusing on the casting structure of a thin slab, and further defining the melting point and thickness of the coating material on the outermost layer, the occurrence of vertical cracks is extremely reduced. It is possible to obtain a thin slab which has a small number of occurrences of lateral cracks and internal cracks.

[Brief description of the drawings]

FIG. 1 shows a relationship between a slab surface vertical crack index and a cooling index obtained by changing coating conditions.

FIG. 2 shows a relationship between a slab surface lateral cracking index and a cooling index obtained by changing coating conditions.

FIG. 3 shows the relationship between a slab internal crack index, a mold level change in a mold, and a cooling index.

──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Atsushi Ishikawa 1 Oshinozu, Oita, Oita City, Oita Prefecture Inside Nippon Steel Corporation Oita Works (72) Inventor Toshiki Yamamoto 20-1 Shintomi, Futtsu City, Chiba Prefecture New Japan (56) References JP-A 7-185747 (JP, A) JP-A 61-195744 (JP, A) JP-A 1-299744 (JP, A) (58) Survey Field (Int.Cl. ⁷ , DB name) B22D 11/06 340 B22D 11/059 110

Claims (1)

(57) [Claims] An endless metal belt that circulates and moves on opposite sides of a long side, and a block that circulates and moves in synchronization with the endless metal belt and opposes on both sides of a short side. Casting equipment has a carbon concentration of 0.08% or more,
In a method for producing thin cast slabs by supplying molten steel of less than 0.25%, a coating material of one or more layers is coated with a melting point of the outermost layer coating material of 950 ° C. or less,
The cooling index R at a position at a depth of 0.2 to 0.7 mm from the surface layer of the obtained slab is 60 μm or more, and
A method for producing a thin cast piece by a belt-type continuous casting method, wherein the casting is performed using the endless metal belt coated so as to have a thickness of 60 or more and 160 or less. R = (d / 191) ^-2.237 where d: average value of dendrite secondary arm spacing (μ
m)