JP2806250B2 - Manufacturing method of continuous cast slab - 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 continuously cast slab having few defects such as corner cracks.

[0002]

2. Description of the Related Art In a continuous casting machine, the shape of a slab which has normally passed through a curved portion is corrected by using a correction band. In this straightening band, a tensile stress acts on the upper surface side of the slab, so that cracks occur when the ductility is low. Generally, the ductility of steel is dependent on temperature, and it is known that there is an embrittlement zone at 750 to 900 ° C. When a slab is manufactured by a continuous casting machine, the corners of the slab are more likely to be cooled than other parts, and enter the embrittlement zone before the other parts. Therefore,
When the slab enters the straightening zone in this state, a crack called a corner crack occurs.

The following technology has been proposed as a measure for preventing corner cracks in the straightening belt. The first technique is based on the control of the secondary cooling spray, and is a method of adjusting the amount of water in the secondary cooling spray and the range of spray water spray on the slab. The simplest method is to reduce the amount of water in the secondary cooling spray so as not to enter the embrittlement region including the slab corner, but this is not always possible because it reduces productivity. Therefore, for example, Japanese Patent Application Laid-Open
In the technique described in JP-B-163559, the cooling of the slab corners is eased by cooling only the central portion without cooling both ends in the width direction of the slab.

[0004] The second technique is a method using a cover. In the technology described in Japanese Patent Application Laid-Open No. 50-110933,
Both ends in the width direction of the slab are covered with covers to prevent spray water from hitting. Further, there is a method in which a heat insulating material is further provided in the cover to keep the corner of the slab warm.

[0005] A third technique is a method of heating a corner portion of a slab. In the technique described in Japanese Utility Model Laid-Open No. 57-97667, a burner is installed in the gap between the guide rolls of the correction belt to heat the vicinity of the corner of the slab. 57-57
In the technique disclosed in Japanese Patent No. 116364, induction heating is performed by disposing an induction coil inserted from a gap between guide rolls of a straightening belt near a corner of a slab.

FIG. 5 is a diagram showing a conventional method for preventing corner cracking. In the figure, 1 is a slab, 5 is a roll, 7 is a bracket, and 8 is a guide rail. The bracket 7 is installed by being inserted into a gap between the rolls 5, and a burner or an induction coil is attached to a tip of the bracket. This method is a method of heating a corner portion of the slab from a thickness direction of the slab.

[0007]

In recent years, a high-speed casting technique with an increased drawing speed has been developed to improve productivity. However, as the drawing speed increases, the thickness of the solidified shell of the slab decreases, and breakout, internal cracking, and the like tend to occur. Therefore, cooling has been strengthened in order to ensure a sufficient thickness of the solidified shell. As a result, the corner portion of the slab is often excessively cooled and is already in the embrittlement region in many cases, causing a problem that cracks (corner cracks) easily occur.

If the conventional technique for preventing corner cracking is applied to such a continuous casting machine for high-speed casting, the following problems arise. First, the method of controlling the secondary cooling spray, cooling only the central part without cooling the both ends in the width direction of the slab, or using a cover is not enough to avoid excessive cooling of the corner part of the slab. It is enough.
Even in this case, the temperature of the corner portion of the slab is lower by 100 to 150 ° C. than that of the center portion in the width direction of the slab.

In these methods, when the slab is strongly cooled, the center in the width direction does not enter the embrittlement zone (1000).
(Around ℃), the corner area is embrittled (900-750 ℃)
Cooling to prevent corner cracking. Therefore, the slab cannot be strongly cooled, and it is necessary to weaken the cooling, and there is a problem that a breakout or the like easily occurs.

Next, in a continuous casting machine intended for high-speed casting, there is a problem that it is difficult to install the heating means itself.
This is because the high-speed casting reduces the thickness of the solidified shell of the slab and easily causes internal cracks in the slab. Therefore, the roll pitch of the guide roll is reduced. As a result of the reduction of the roll pitch, it becomes difficult to insert a burner or an induction heating device from the gap between the rolls. In addition, since the roll diameter is reduced with a reduction in the roll pitch, the size of a portion surrounded by the surface of the roll and the surface of the slab is reduced, and it is difficult to install a burner or an induction heating device in this portion. .

In general, a method of compressing the interval between rolls in accordance with shrinkage due to solidification of the slab to adjust the cross-sectional shape of the slab has been performed. As a result, the heat transfer due to the contact between the slab and the roll is enhanced. In this case, since the cooling of the corner portion of the slab is also enhanced, the risk of being cooled to the embrittlement region increases.

The present invention solves the above problems and provides a method for producing a continuous cast slab having a small number of defects such as corner cracks, especially a high speed cast slab.

[0013]

According to the present invention, there is provided a continuous casting method in which a slab having an unsolidified portion is supported by guide rolls having a predetermined roll interval and solidified while being pulled out.
(B) In the secondary cooling zone, the roll gap expansion section in which the roll gap of the guide rolls is gradually or stepwise expanded with the movement of the slab, the roll gap holding section in which the roll gap is kept substantially constant, and the movement of the slab In addition, three sections of a roll interval reduction section for gradually reducing the slab are provided, and (b) the slab is sequentially cooled in these sections. (C) Before the slab exits the roll interval holding section, Heat the corner from the side of the slab.
A method for manufacturing a continuously cast slab, wherein a curved slab is straightened in a roll interval holding section or a roll interval reducing section.

The invention according to claim 2 is characterized in that (e) using a vertical bending type continuous casting machine, and (f) starting from a time when the slab is located at a position before the roll interval enlarging section, to a corner on the reference surface side of the slab. 2. The method for producing a continuous cast slab according to claim 1, wherein the portion is heated from the side of the slab, and (g) thereafter, bending deformation for bending the slab is started at a position before the roll gap enlarging section. It is.

[0015]

The present invention is characterized in that in order to prevent corner cracking of a continuously cast slab, roll interval control is performed on some of the guide rolls in the secondary cooling zone. The invention of claim 1 will be described along the movement of the slab.

First, a slab having an unsolidified portion therein enters a roll interval enlarged section. At this time, the cross-sectional shape of the slab is basically the same as the cross-sectional shape at the mold outlet. In the process of moving in this section, the slab expands due to the hydrostatic pressure applied to the unsolidified portion inside because the interval between the rolls is gradually increased. At this time, the slab expands so that the cross-sectional area of the internal unsolidified portion increases. At this time, when the cross section is flat like a slab, the slab expands so that its thickness direction increases.

At this time, since the side surface (short side) of the slab is already solidified, the dimension in the thickness direction does not change at both ends in the width direction of the slab, ie, around the corner. As a result, the periphery of the corner portion does not come into contact with the guide roll, so that cooling by contact heat transfer with the guide roll is prevented.

Next, in the roll interval holding section, since the roll interval is substantially constant, the expansion of the slab stops, and the cross-sectional shape of the slab becomes the same as when the roller exits the roll interval expanding section. As a result, the state in which the corner portion of the slab does not come into contact with the guide roll continues, so that the corner portion of the slab is re-heated by the heat flow from the periphery and the temperature difference from the periphery is reduced.

Since the heating of the corner of the slab is started before the slab leaves the roll interval holding section,
The corners are heated in a recuperated state. Therefore, the temperature of the corner easily rises and becomes higher than the embrittlement zone.

Next, the curved slab enters the straightening zone and is straightened straight. Here, the corner portion undergoes tensile deformation, but since the temperature is higher than the embrittlement region, corner cracking can be prevented. Here, starting the heating of the corner portion before the slab exits the roll interval holding section,
This is because the expanded slab is compressed after the slab enters the roll interval reduction section, and the corner may come into contact with the guide roll.

The cross section of the slab expands in the process of moving to the roll interval holding section. However, the roll interval is gradually reduced in the roll interval reducing section, so that the slab is finally compressed and has a desired cross sectional shape. It is adjusted to.

The invention of claim 2 is a method in which the invention of claim 1 is applied to a case where a vertical bending type continuous casting machine is used. In the vertical bending type continuous casting machine, the slab that has exited the mold is flat, and needs to be bent at the bending deformation zone in order to pass through the curved secondary cooling zone. Therefore,
In addition to the first aspect, heating and bending deformation of the corner portion of the slab are further performed.

First, by heating a corner portion of the slab on the reference surface side (the lower surface after casting), the temperature of the corner portion rises. In the bending deformation zone, the slab temperature is higher than in the straightening zone, so that the temperature drop in the corners is smaller than in the straightening zone. Therefore, the temperature of the corner easily rises by heating and becomes higher than the embrittlement zone.

Next, bending deformation is applied to the slab in this state. Although tensile stress acts on the reference surface side of the slab, corner cracks do not occur because the corners are hotter than the embrittlement zone. Since the corner heating and the bending deformation start at a position before the roll gap expanding section, the corner portion is restrained by the guide roll. As a result, the bending deformation force is transmitted to the slab side surface portion adjacent to the corner portion, and the bending deformation is performed smoothly. Other operations are the same as those of the first aspect.

[0025]

FIG. 1 is a view showing a method of setting a roll interval between guide rolls in an embodiment in which the present invention is applied to a vertical bending continuous casting method. In the figure, the horizontal axis indicates the distance L from the meniscus, the vertical axis indicates the roll interval, A indicates the position of the bending deformation zone, and B indicates the position of the correction zone. In the figure, indicates a roll interval expanding section in which the roll interval is increased, indicates a roll interval holding section in which the roll interval is constant, and indicates a roll interval reducing section in which the roll interval is reduced.

In the roll interval expanding section, the roll interval is expanded by 2 mm from 228 mm to 230 mm.
As a result, the other portion of the slab is 1 mm thicker on one side than the corner of the slab. Thereby, even if the roll has some unevenness (within 1 mm), the corner portion of the slab does not contact the roll.

In the case of slab after continuous casting, which is not reheated in a heating furnace, and is directly rolled in a rolling mill, the roll interval is also increased. However, in this case, the amount of expansion of the roll interval is considerably larger than that of the present invention, for example, as much as 5 mm from 228 mm to 233 mm. In this case, the expansion amount of the roll interval is 2.2% with respect to the slab thickness.

In the embodiment of the invention, the amount of expansion of the roll interval is smaller than this, that is, 2 mm. The amount of expansion of the roll interval is 0.88% with respect to the slab thickness. The amount of expansion of the roll interval is preferably 1% or less with respect to the slab thickness, but may be smaller. Since irregularities due to abrasion, bending and the like of the guide rolls are usually controlled to be smaller than 0.5 mm, it is sufficient that the roll gap expansion amount of the present invention is about 0.5 mm on one side and about 1 mm on both sides. In such a case, the amount of expansion of the roll interval is 1% of the slab thickness.
The following may be used.

FIG. 2 is a schematic diagram showing a state of a change in a slab cross section in each region shown in FIG. In the figure, 1 indicates a slab (cross section), and 5 indicates a roll. Also,"
"Before" is "Before the roll interval expansion section", "
”And“ ”are“ roll interval expansion section and holding section ”,
The relationship between the roll and the slab cross section in the “roll interval reduction section” is shown.

First, before the roll interval expansion section,
The slab surface, including the corners, is in contact with the roll. However, in the next roll interval expansion section, the slab expands in the thickness direction, and the corner portion does not come into contact with the roll. This state does not change even in the roll interval holding section. Finally, in the roll interval reduction section, the slab is compressed in the thickness direction, and accordingly, the difference in thickness from the corner portion is reduced.

FIG. 3 is a diagram showing a change in the temperature of the corner portion of the slab in the embodiment of the invention and the comparative example. In the figure,
The horizontal axis is the distance L from the meniscus, the vertical axis is the temperature T of the corner of the slab, the broken line a is the temperature transition of the embodiment of the invention, the broken lines b and c are comparative examples, and b is the case where only the roll interval control is performed. c shows the temperature transition when neither heating nor roll interval control is performed, A shows the installation position of the heating means in the bending deformation zone, and B shows the installation position of the heating means in the straightening band.

When neither heating nor roll interval control was performed (polyline c), the temperature T of the slab corner dropped to 850 ° C. in the bending deformation zone A, and was already in the embrittlement zone (900 to 75 ° C.).
0), and the temperature is lowered to 710 ° C. in the correction zone B. On the other hand, when only the roll interval control is performed (polyline b), the temperature of the corner portion does not change in the bending deformation zone A immediately after the start of the roll interval control. Approximately 5
0 ° C temperature is high.

In the case of the embodiment of the present invention (polyline a), the temperature T of the slab corner is higher than the embrittlement zone at 950 ° C. in the bending deformation zone A, and is higher than the embrittlement zone again at 930 ° C. in the correction zone B. ing. Here, immediately before the heating of the correction band B is started, the temperature of the corner portion has decreased to 850 ° C., but this portion is inside the curved band, and in the curved band, the tensile stress is applied to the corner portion of the slab. Does not work,
No problem.

When only the roll interval control is performed, the temperature drop at the corner of the slab is about 20 m from the corner of the corner.
The area was as narrow as about m. Since the occurrence of corner cracks is also concentrated in this region, it can be seen that the heating device used in the present invention only needs to be able to heat a portion approximately 20 mm from the corner of the corner of the slab. It is desirable from the viewpoint of effective utilization of energy that the region to be subjected to induction heating is also concentrated in this portion.

In this embodiment, the heating of the corner portion is started before the roll interval enlarging section and the bending deformation is performed at the beginning of the roll interval enlarging section. It may be performed within or before the section. However, after fully expanding the slab section,
Since the periphery of the corner cannot be restrained by the roll, it is desirable that the bending deformation is completed at least at the initial stage of expansion of the slab cross section.

In this embodiment, the case where a vertical bending type continuous casting machine is used is shown. However, substantially the same effect can be expected when a curved type continuous casting machine is used. Since the curved continuous casting machine has no bending deformation zone as compared with the vertical bending type, it is only necessary to perform corner heating only on the straightening zone. In this case, since there is no corner heating corresponding to the bending deformation zone, the heating can be easily performed by increasing the capacity of the heating device or starting the corner heating early to compensate for the corner heating.

FIG. 4 is a diagram showing an example of an induction heating device used for heating a corner portion. In the figure, 1 is a slab, 2 is a heating means, 3 is an adjusting mechanism, 4 is a power supply cable, 5 is a guide roll, 6 is a split bearing, and 21 is an induction heating device.

Here, in order to prevent break-out or internal cracking of the slab, the diameter of the guide rolls 5 is reduced by employing split bearings 6 and their roll pitches are reduced. Even in such a slab support device using a small-diameter guide roll, the slab corner portion can be heated by using the induction heating device 21 as the heating means 2 and facing the side surface of the slab 1 further. Is possible.

The induction heating device 21 is constituted by an induction coil having one turn.
5 kHz, capacity 300 kW. Desirable casting conditions for this induction heating device are slab thickness 220, 25
0, 300 mm, slab width 1600-2100 mm, casting speed 1.2-2.0 m / min.

In the above embodiment, the induction heating device is used for both the bending deformation band and the correction band as the heating means 2, but these may be burners or the like. However, it is preferable to use an induction heating device because there is a risk that the burner flame may hit the split rolls and split bearings if the burner combustion adjustment is not always taken care of. In particular, for the bending deformation zone, the use of a small-diameter roll is inevitable for high-speed casting, and as a result, the guide roll and the like are easily hit by the flame of the burner. Therefore, it is preferable to use an induction heating device.

[0041]

According to the present invention, the unsolidified slab is expanded by controlling the roll interval between the guide rolls to prevent the contact heat transfer between the corners and the guide rolls, so that the temperature drop in the corners is reduced. By heating after the corner portion is sufficiently double-heated, the temperature of the corner portion easily rises and becomes higher than the embrittlement region, so that corner cracking can be prevented. When the vertical bending type continuous casting machine is used, the corner portion is heated even in the bending deformation zone, so that the ductility of the corner portion is recovered and the corner crack can be prevented.

[Brief description of the drawings]

FIG. 1 is a diagram showing a method of setting a roll interval between guide rolls in one embodiment of the present invention.

FIG. 2 is a schematic diagram showing a state of a change in a slab cross section.

FIG. 3 is a diagram showing a change in temperature at a corner of a slab in one embodiment of the present invention.

FIG. 4 is a diagram illustrating an example of an induction heating device used for a heating unit.

FIG. 5 is a diagram showing a conventional corner crack preventing method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Slab 2 Heating means 5 Guide roll Roll interval expansion section Roll interval holding section Roll interval reduction section A Bending deformation zone B Straightening zone

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yuichi Yamaoka 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Taizo Sera 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Inside Kokan Co., Ltd. (72) Inventor Minoru Takada 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Kokan Co., Ltd. (56) References JP-A-2-211954 (JP, A) JP-A-62-13248 ( JP, A) JP-A-61-154750 (JP, A) JP-A-7-241656 (JP, A) JP-A-55-42109 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , (DB name) B22D 11/128 B22D 11/128 310 B22D 11/12 B22D 11/16

Claims (2)

(57) [Claims] 1. In continuous casting in which a slab having an unsolidified portion is supported by guide rolls having a predetermined roll interval and solidified while being drawn, (a) the roll interval of the guide rolls is set in a secondary cooling zone. (B) slab is provided with three sections: a roll gap expansion section that is gradually or stepwise enlarged with the movement, a roll gap holding section that is substantially constant, and a roll gap reduction section that is gradually reduced with the movement of the slab. Are sequentially cooled in these sections, and (c) before the slab exits the roll interval holding section, the corner portion on the upper surface side of the slab is heated from the side of the slab. A method for producing a continuously cast slab, wherein a curved slab is straightened in a reduced interval section. (E) using a vertical bending type continuous casting machine, and (f) changing the corner of the slab from the side of the slab from the side of the slab from the time before the slab is at a position before the roll gap enlarging section. The method for producing a continuous cast slab according to claim 1, wherein the slab is heated, and thereafter, bending deformation for bending the slab is started at a position before the roll gap enlarging section.