JPH10272543A - Method for continuously casting steel having high hot strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the large variation of molten metal surface in a mold, the lowering of defective quality and yield and the development of operational trouble just after starting the casting by casting under of lowering cooling capacity over till a bending part or a straightening part in the secondary cooling zone at the initial stage of casting of a kind of steel having high hot strength. SOLUTION: Until the tip part of a cast slab having high hot strength, such as SUS304, SUS420J1, reaches the bending part in the secondary cooling zone, the cooling capacity from this vertical part to the bending part is lowered. For example, the secondary cooling water to the tip part of cast slab is not sprayed or is sprayed on a little quantity to near the bending part. By this method, the surface temp. at the tip part of cast slab reaching the bending part is raised and the hot strength becomes low, and since the cast slab can sufficiently be bent at the bending part and the straightening part, it can be prevented to develop the variation of the molten metal surface caused by the collision to a roll at the bending part or the straightening part. Therefore, the breakout caused by the variation of the molten metal surface and the increase of mending to the tip part of cast slab or scrap-down, etc., can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a continuous casting method in a continuous casting machine, especially a steel having a hot strength of not less than 60 MPa in a tensile test at 900 ° C. and a strain rate of 10 ^-2 sec ^-1. For example, the present invention relates to a continuous casting method for preventing a quality defect, a decrease in yield, and a breakout due to a change in a molten metal surface when continuously casting steel having a high hot strength such as SUS304 steel, SUS420J1 steel, or SUS420J2 steel.

[0002]

2. Description of the Related Art Generally, when steel is continuously cast by a continuous casting machine, in an initial stage, a dummy bar head of a dummy bar is arranged at the bottom of a mold to close the bottom, and then molten steel is poured into the mold. In the mold for a certain period of time, the water pool is kept, and at this time the cold material charged before casting and the cooling from the mold solidify the molten steel in the mold and solidify shell on the surface in contact with the dummy bar head and the surface in contact with the mold Is formed to a certain thickness, and then a dummy bar is pulled out, whereby a slab is pulled out along with the dummy bar. And the subsequent slab is
The solidified shell is grown to a certain thickness by cooling in the mold, and after it is drawn out of the mold, it is completely solidified by the secondary cooling water from the spray nozzle provided immediately below the mold. proceed.

[0003] By the way, continuous casting machines used for continuous casting of such steel are roughly classified into: (1) The casting machine is arranged so as to be in a vertical direction until cutting a solidified slab from a mold. Vertical continuous casting machine, (2) All-bending continuous casting machine in which the height from the mold to the specified length is arranged in a curved shape, and the machine height is reduced, (3) The length from the mold to the predetermined length is vertical, and then the bent part, A vertical section with curved sections and straightening sections of constant R
Bending type continuous casting machines are known. Among these continuous casters, the longer vertical portion has less non-metallic inclusion defects in the obtained slab due to the floating effect of the inclusions. Therefore, the vertical type continuous caster is most excellent in terms of quality. But,
Vertical continuous casting machines are expensive, construction costs are high,
Due to low productivity, the newly installed continuous casters are currently mainly of the fully curved type or the vertical bending type.

[0004] In recent years, with the spread of the continuous casting machine and the promotion of technical development in parallel with it, continuous casting of various types of steel, which has been difficult until now, has been attempted. Therefore, the inventor conducted continuous casting of various steels using the above-described vertical bending type continuous casting machine, and in the case of a steel type having a high hot strength such as SUS304, SUS420J1 or SUS420J2, immediately after the start of casting. In the meantime, the level of the molten metal in the mold was remarkably fluctuated. This fluctuation of the molten metal surface causes the mold powder covering the molten metal surface in the mold to be entangled in the molten steel, causing defects such as slag of the slab, or a decrease in yield due to streaks generated on the slab surface. In addition to causing the occurrence of balding,
It causes serious operation troubles such as breakout of the slab in or under the mold.

As a conventional technique for preventing the fluctuation of the molten metal level, as disclosed in Japanese Patent Application Laid-Open No. Hei 4-17963, a rapid flow of molten steel and a surface flow in the mold are generated by generating a magnetic field in the mold. There is known a method in which the temperature is controlled by braking with a Lorentz force to prevent the fluctuation of the molten metal level.

[0006]

SUMMARY OF THE INVENTION The above-mentioned JP-A-4-14-1
The technology disclosed in No. 7963 is a technology for preventing the level change of the molten metal occurring during the steady casting.
A satisfactory effect was not sufficiently obtained.

[0007] The present invention advantageously solves the above-mentioned problems, and when continuously casting a steel type having a high hot strength such as SUS304, SUS420J1 or SUS420J2, the molten metal surface in the mold immediately after the start of casting. To prevent remarkable fluctuations, and consequently defects such as slag of the slab due to entanglement of the mold powder covering the molten metal surface in the molten steel, decrease in yield due to streaks generated on the slab surface, and scabble etc. It is an object of the present invention to propose a continuous casting method for steel capable of preventing serious operation troubles such as occurrence of cracks and breakout of a slab in or under a mold.

[0008]

Means for Solving the Problems Now, the inventor has conducted a detailed investigation of the cause of the fluctuation of the molten metal level at the time of the initial casting as described above, and has found the following. That is,
For steel types such as SUS304, SUS420J1 and SUS420J2, in addition to the high hot strength, the vicinity of the tip of the slab, especially in the area in contact with the dummy bar head, is shaped to engage with the dummy bar head. , So that it is easy to be supercooled, and hence the strength is increasing. Therefore, in the bent portion or the straightening portion of the secondary cooling zone, the tip end of the slab cannot sufficiently bend along the arc or the straightening portion of the bent portion and collides with the roll. It has been clarified that this is the cause of the fluctuation of the molten metal level.

In order to solve this problem, further intensive studies have been made, and as a result, the supercooled slab tip (start bottom) is easily deformed by the bent or straightened portion. Therefore, it is effective to reduce the cooling capacity in these regions while the tip passes from the vertical portion to the bent portion, specifically, the secondary cooling applied to the slab tip. It was found that controlling the amount of cooling water by controlling the timing at which water was started to spray was good. Also, in the curved continuous casting machine, a phenomenon that the tip of the slab collides with the roll in the curved portion or the straightening portion is observed, and in this case, the cooling ability from directly below the mold to the curved portion or the straightening portion may be reduced. It turned out to be effective.

According to the present invention based on the above findings, when casting a steel type having a high hot strength by using a continuous casting apparatus having a curved portion in a secondary cooling zone, a curved portion or straightening portion is formed immediately below a mold in an early stage of casting. Continuous casting method of steel type with high hot strength characterized by casting with reduced cooling capacity over the part, and vertical bending type continuous having vertical part, bent part and bent part in secondary cooling zone When casting a steel type with high hot strength using a casting device, bend from this vertical part until the tip of the slab formed in contact with the dummy bar head in the early stage of casting reaches the curved part of the secondary cooling zone. A continuous casting method for a steel type having a high hot strength, characterized in that casting is performed in a state in which the cooling capacity over a part is reduced. The means for reducing the cooling capacity include stopping the cooling at the bending part, and when the steel type with high hot strength is not less than 60 MPa in the tensile test at 900 ° C with a strain rate of 10 ^-2 sec ^-1. The effect of the present invention is remarkably exhibited when the steel has a hot strength of

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the continuous casting method of the present invention, SU
When casting a steel type with high hot strength such as S304, SUS420J1 or SUS420J2, reduce the cooling capacity from this vertical part to the bent part until the slab tip reaches the curved part of the secondary cooling zone. (For example, the secondary cooling water applied to the tip of the slab is not applied to the vicinity of the bent part or is reduced to a small amount.) Therefore, the surface temperature of the tip of the slab reaching the bent part is increased compared to the conventional case. However, the hot strength was reduced, and it became possible to sufficiently bend at the bent portion and the straightening portion, so that it hit the roll at the bent portion and the straightening portion as in the past, and as a result, the fluctuation of the molten metal surface occurred Can be prevented.

Hereinafter, the continuous casting method of the present invention will be described more specifically with reference to the drawings. FIG. 1 shows that according to the invention,
Examples of the cooling pattern of the secondary cooling zone are shown by. 1 is a vertical bending type continuous casting machine, in which a plurality of rolls 2 for guiding a slab are disposed immediately below a mold 1, and a vertical portion, a bent portion, and a fixed portion are arranged from the mold side. A secondary cooling zone configured in the order of the R portions is formed. By injecting cooling water toward the slab by spraying from the gap between these rolls, secondary cooling of the slab is performed, and in the illustrated vertical bending type continuous casting machine, the mold spray just below the mold, It has a vertical part spray, a bent part spray and a constant R part spray.

A continuous casting method in a vertical bending type continuous casting machine as shown in FIG. 1 will be described with reference to FIG. 2. First, as shown in FIG.
The dummy bar 5 is arranged so that is inserted. After the bottom of the mold 1 is closed with the dummy bar head 5, molten steel is injected into the mold 1 and a solidified shell is formed to a certain thickness in the mold 1 by removing heat from the surroundings. The dummy bar 5 is pulled out and driven. As a result, as shown in FIG.
Is drawn out of the mold 1 and cooled by the mold spray immediately below the mold 1 and the secondary cooling water in the secondary cooling zone, whereby the slab 3 is completely solidified. The secondary cooling zone includes a vertical portion, a bent portion, a curved portion (constant R portion), a straightening portion, and a horizontal portion. After the dummy bar 5 and the slab 3 are pulled out in the vertical direction, the bending unit 6 And it is pulled out in an arc state.

In the initial stage of casting a steel having a high hot strength by such a vertical bending type continuous casting machine, in the initial stage, the vicinity of the tip of the slab 3 is shaped so as to engage with the dummy bar head 4. Because of this, it is easy to be supercooled and the strength is higher, so in the conventional casting method, it hit the roll at the bent part or straightening part of the secondary cooling zone, causing the fluctuation of the molten metal level Has already been described.

Therefore, according to the present invention, casting is performed in a state where the cooling capacity from the vertical portion to the bent portion is reduced until the slab tip reaches the curved portion of the secondary cooling zone. For example, cooling water patterns such as 1 to 4 in FIG. 1 are selected according to the magnitude of the hot strength of the steel type to be cast, the casting speed, the magnitude of the curvature of the bent portion, and the like.

The cooling water pattern 1 shown in FIG. 1 indicates that the tip of the slab 3 moves as the continuous casting proceeds, and the mold spray is not used until the tip of the slab passes the bent portion. Is a cooling water pattern in which the secondary cooling water is not discharged even from the spray of No. 1. The cooling water pattern 2 in FIG.
As the tip of the slab 3 moves as the continuous casting proceeds, cooling water is discharged from the mold spray in the same manner as before, and the secondary cooling water in other parts passes through the bent part at the tip of the slab 3 This is a cooling water pattern in which the cooling water is not discharged until it is discharged. The cooling water pattern 3 in FIG. 1 shows that the cooling water from the mold spray, the vertical part spray, the bending part spray, etc., is transferred to the slab 3 in response to the movement of the tip of the slab 3 as the continuous casting proceeds. This is a cooling water pattern that is discharged when the tip portion passes through the portion. The cooling water pattern of 4 in FIG. 1 shows that the cooling water is discharged from the mold spray in the same manner as in the past, and the vertical spray and the bending spray are applied to the movement of the tip of the slab 3 as the continuous casting proceeds. The secondary cooling water has a cooling water pattern that is discharged when the tip end of the slab 3 passes through the portion.

With these cooling water patterns, compared with the conventional cooling water pattern shown by the broken line in the figure, the cooling ability to the tip of the slab 3 is relaxed. Cooling is prevented, and the hot strength is reduced, so that it can be sufficiently bent at a bent portion or a straightened portion.

[0018] Of the four types of cooling water patterns described above, one pattern has the lowest cooling capacity.
The pattern may be appropriately selected according to conditions such as steel type.
Further, the present invention is not limited to the above-mentioned four types of patterns, and various modifications are possible without departing from the gist of the present invention.

[0019]

EXAMPLE A SUS martensitic steel was cast according to the pattern 2 shown in FIG. That is, SUS martensitic steel (C: 0.025 wt%, Si: 0.25 wt%, Mn: 0.45 wt%
%, P: 0.015 wt%, S: 0.010 wt%, Al: 0.020 wt%
%, Ni: 4.0 wt%, Cr: 13 wt% Mo: 1.0 wt%) and a continuous casting machine (bending part radius: 9.6 m, straightening part position: 16.2-19.5)
m, vertical length: 2.5 m, bending position: 2.5 m to 4.3 m
, Mold: 260 mm thick x 730 mm wide, initial speed 0.5 m / min)
When the continuous casting was started and the secondary cooling water other than the mold spray was not discharged until the dummy bar head 4 reached the vicinity of the constant R portion, the tip of the slab 3 was bent at the bent portion of the secondary cooling zone. However, the situation of colliding with the roll and causing a change in the level of the molten metal was eliminated, and good results were obtained.

FIG. 3 is a time series chart showing the level of the molten metal level when the conventional cooling water pattern is used and when the cooling water pattern in this embodiment is used. From the figure, the conventional method (FIG. 1A) shows that the molten metal level fluctuates at a timing corresponding to the roll position of the bending portion, whereas the cooling water pattern according to the present invention (FIG. 2B)
In), there was no change in the bath level.

In this embodiment, an embodiment in which SUS martensitic steel is continuously cast has been described. However, the present invention is not limited to the embodiment, and the hot-strength is not limited thereto. 1 can be used for the steel type having a large value, and the cooling method for the patterns 2 to 4 or other patterns shown in FIG. 1 can be used for a steel type having a lower hot strength than the steel type having a lower hot strength. Needless to say.

[0022]

According to the continuous casting method of the present invention, in the initial casting stage of steel in a continuous casting machine, when casting a steel type having a high hot strength, the secondary cooling water applied to the tip of the slab is subjected to a constant R portion. Since a small amount is not applied to the vicinity, or a small amount is used, it is possible to prevent the problem that the tip end of the slab collides with the roll at the bent portion to cause a change in the molten metal level. As a result, it is possible to prevent the fluctuation of the molten metal level, so that operation troubles such as breakout due to the fluctuation of the molten metal level, an increase in the care of the slab tip or scrap drop caused by the fluctuation of the molten metal level, or a large fluctuation of the molten metal level Accordingly, there is an industrially remarkable effect that defects such as inability to cast depending on the type of steel can be prevented.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a cooling water pattern according to the present invention.

FIG. 2 is an explanatory view of an initial casting method in a vertical bending type continuous casting machine.

FIG. 3 is a diagram showing a time-series change of a molten metal level in a conventional example and an example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold 2 Roll 3 Slab tip 4 Dummy bar head 5 Dummy bar 6 Bending unit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Gentatsu Sugisawa, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Chiba Works, Ltd. Chiba Works (72) Inventor Hiroshi Sekiguchi 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Kawasaki Steel Corporation Chiba Works (72) Inventor Hiroshi Nishikawa 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Corporation Chiba Works

Claims (4)

[Claims] In casting a steel type having a high hot strength using a continuous casting apparatus having a curved portion in a secondary cooling zone,
A continuous casting method for a steel type having a high hot strength, wherein casting is performed in a state in which cooling ability from immediately below a mold to a curved portion to a straightening portion is lowered in an early stage of casting. 2. Casting a steel type having a high hot strength using a vertical bending type continuous casting apparatus having a vertical portion, a bent portion, and a curved portion in a secondary cooling zone, in contact with a dummy bar head at an early stage of casting. Until the tip of the cast slab reaches the curved part of the secondary cooling zone, it is characterized by casting with reduced cooling capacity from this vertical part to the bent part, characterized by a high hot strength steel type Continuous casting method. 3. The continuous casting method for a steel type having a high hot strength according to claim 2, wherein the decrease in the cooling capacity includes stopping the cooling at the bent portion. 4. The steel type having a high hot strength is a steel having a hot strength of 60 MPa or more in a tensile test at a strain rate at 900 ° C. of 10 ⁻² sec ^−1. , 2 or 3
A continuous casting method for a steel type having a high hot strength as described above.