JP5999294B2 - Steel continuous casting method - Google Patents

Description

  The present invention relates to a steel continuous casting method in which occurrence of surface cracks of a slab in continuous casting is suppressed.

  Low alloy steels containing alloy elements such as Cu, Ni, Nb, V and Ti have been applied to thick steel plates in particular for the purpose of improving the mechanical properties of the steel plates. When such a low alloy steel is cast using, for example, a vertical bending type continuous casting machine, the four corners of the rectangular cross section (hereinafter referred to as the corner portion) perpendicular to the casting direction of the slab at the straightened portion or the bent portion of the slab are obtained. Stress) and surface cracks, particularly cracks at the corners are likely to occur. This corner crack is likely to cause a surface flaw of the thick steel plate, and causes a decrease in the yield of the steel plate product.

That is, the slab of low alloy steel has a marked decrease in hot ductility at a temperature in the vicinity of the Ar ₃ transformation point where the solidification structure transforms from an austenite phase to a ferrite phase. Furthermore, in the slab of low alloy steel, AlN, NbC, etc. are precipitated at the austenite grain boundaries during the secondary cooling, and are easily embrittled. For this reason, cracks are likely to occur on the surface of the slab, especially on the corner portion where stress is applied.

  Therefore, in the continuous casting process, in order to prevent the above-described corner cracking, it is generally performed to control the slab surface temperature by secondary cooling to control the slab solidified structure to a structure that is difficult to break.

For example, in Patent Document 1, secondary cooling of a slab is started immediately after the slab is drawn out of a rectangular mold, and after the surface temperature of the slab is once cooled to a temperature lower than the Ar ₃ transformation point, Ar ₃ When reheating to a temperature exceeding the transformation point and then straightening the slab, the time required to maintain the slab surface temperature below the Ar ₃ transformation point and the lowest temperature that the slab surface temperature can reach are appropriately set. By making the range, a technique is disclosed in which the solidified structure from the slab surface to a depth of at least 2 mm is a mixed structure of ferrite and pearlite in which the austenite grain boundary is unclear.

Patent Document 2 discloses that when the solidified shell thickness is 10 mm or more and 15 mm or less, the primary cooling by the mold is finished and the secondary cooling is started, and the surface temperature of the entire surface of the slab is within 2 minutes from the exit of the mold. In the meantime, a technique is disclosed in which the temperature is once lowered to a range of 600 ° C. or higher and below the Ar ₃ point, and secondary cooling is performed so that both the slab surface temperature in the bent portion and the slab surface temperature in the straightened portion become 850 ° C. ing.

Patent No. 3702807 Patent No. 3058079

However, the above-described conventional technology has the following problems.
That is, in the techniques described in Patent Document 1 and Patent Document 2, there is a concern about the influence of dripping water flowing through the slab after being sprayed from the secondary cooling spray to the slab. In particular, when the casting speed becomes slow, dripping water affects the cooling of the slab surface, and it may be difficult to quantitatively control the slab surface temperature by, for example, heat transfer analysis.

Furthermore, in the technique described in Patent Document 2, in order to reduce the temperature of the entire surface of the slab to the Ar ₃ transformation point or less, a large amount of spray water must be injected. If the casting thickness is large, a larger amount of spray water is required, but if too much spray water is sprayed, temperature fluctuations are likely to occur in the width direction of the slab, and internal cracks occur under the surface of the slab. It will be a concern.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to form a surface crack of a slab, which has not been sufficiently solved only by controlling the slab structure by secondary cooling, in an appropriate shape. An object of the present invention is to provide a high-quality slab that is reliably suppressed by controlling the temperature of the slab corner portion by secondary cooling while using a mold having a casting space, and that is particularly free from corner cracks.

The gist configuration of the present invention is as follows.
(1) In a continuous casting method in which molten steel is charged into a mold and a slab is drawn directly from the mold,
At the four corners of the rectangular space defined by the pair of mold long sides and the pair of mold short sides, the ratio b / a of the length b on the mold short side to the length a on the mold long side is 3.0 or more and 6.0. Using the casting mold, which has the casting space removed in the shape of a right triangle that becomes
Before reaching the bending correction point from directly below the mold, the surface temperature of at least the corner portion of the slab is once lowered to Ar ₃ point or less, and then at least the surface temperature of the corner portion is set to 800 ° C. or higher. A method for continuous casting of steel, wherein the bending correction point is passed at 800 ° C or higher.

(2) The steel continuous casting method according to (1), wherein the ratio b / a is greater than 4.0.

(3) The length a on the mold long side is 4 to 6 mm, and the length b on the mold short side is 12 to 36 mm, according to (1) or (2), Steel continuous casting method.

  According to the present invention, by using a mold in which a casting space of an appropriate shape is partitioned, the temperature of the slab corner portion is controlled by secondary cooling, thereby preventing corner cracking of the continuous cast slab, and high quality slab Can be provided.

It is a figure which shows a continuous casting machine. It is a schematic diagram which shows the crystal structure of a slab corner part. It is a schematic diagram which shows the crystal structure of a slab corner part. It is a schematic diagram which shows a casting_mold | template. It is a graph which shows the relationship between the chamfering shape in a casting_mold | template, and the stress in a slab corner part.

Hereinafter, the continuous casting method of the present invention will be described in detail with reference to the drawings.
Now, the molten steel is continuously cast using a vertical bending type continuous casting machine as shown in FIG. 1, for example, but at this time, surface cracks are not induced at the corner of the slab, particularly during straightening at a bending straightening point. Therefore, it is important to use a mold in which a casting space of an appropriate shape is partitioned and to pass an appropriate cooling pattern in a cooling zone immediately below the mold.

  In FIG. 1, reference numeral 1 denotes molten steel charged in the ladle 2. The molten steel 1 is supplied into a water-cooled mold 6 from a ladle 2 through a long nozzle 3, a tundish 4 and an immersion nozzle 5. The molten steel 1 cooled by the water-cooled mold 6 is guided to the outlet side of the mold 6 while forming a solidified shell, and is pulled out of the mold 6, and further cooled in the secondary cooling zone 7 immediately below the mold 6 to be solidified. Promoted shell growth. On the exit side of the secondary cooling zone 7, the slab is forced to bend and guided in the horizontal direction, and then the bending is corrected in the drawing straightening zone (bending portion) 8 to form the continuous casting slab 9.

  Here, the inventors observed surface cracks on the slab cast by the vertical bending die continuous casting machine shown in FIG. The cracks of the slab are concentrated at the lower surface corner and the vicinity thereof (see FIG. 2). In addition, the lower surface side of a slab means the outer side of the bending of the curved belt of a vertical bending continuous casting machine, ie, the long side surface side which becomes a lower surface in a horizontal belt. When the structure of this crack was observed by etching, it was found that cracks occurred along the prior austenite grain boundaries, as schematically shown in FIG. From these investigation results, it was considered that corner cracks occurred on the bottom surface of the slab due to the stress load at the bent part, and experiments were conducted to variously change the secondary cooling conditions.

That is, when an experiment using heat transfer analysis was conducted under various secondary cooling conditions, the surface temperature of the slab corner was temporarily reduced below the Ar ₃ point from immediately under the mold until entering the bent part. Then, it was found that if the surface temperature of the slab corner portion is controlled by secondary cooling before entering the bent portion, cracking of the slab corner portion is reduced.
However, some of the slabs still have corner cracks on the lower surface side, and when the solidification structure around these corner cracks is observed, the slab surface layer is made of prior austenite grains as schematically shown in FIG. Although a mixed structure of ferrite and pearlite with unclear boundaries is being obtained, the prior austenite grain boundaries remain in part. Then, it was found that corner cracks occurred along the remaining old austenite grain boundaries.

  Furthermore, when this phenomenon was investigated and arranged using water model experiments and numerical analysis techniques, it was found that the secondary cooling water drooped. That is, after the secondary cooling water is sprayed from the spray toward the slab, a part of the water flows along the slab surface and becomes so-called dripping water, contributing to cooling of the slab. Since the amount of dripping water changes when the casting conditions such as casting speed, casting width and slab surface temperature change, it is very difficult to accurately evaluate the influence of dripping water. Such dripping water affects the slab temperature, and as a result of cooling the slab more than expected, old austenite grain boundaries remain in a part of the solidified structure, and the old austenite grains are accompanied by the stress load of the bending part. It was thought that cracks along the boundary occurred.

Therefore, if the slab temperature can be controlled taking into account the influence of dripping water, it is possible that the solidified structure can be perfected, but a spray based on a very detailed analysis is possible. It is assumed that control and equipment maintenance are required, and it is not realistic in industrial scale manufacturing.
In general, the vertical bending type continuous casting machine is a casting machine having a short vertical part length of, for example, about 3.5 m before entering the bending part. Thus, in a continuous casting machine with a short distance to enter the bending portion, when the slab is excessively cooled by the influence of dripping water or the like when the temperature is once lowered to the Ar ₃ point or less, the bending is thereafter performed. It is difficult to earn time for reheating before entering the part, and the solidified tissue is assumed to be incomplete.

Under such circumstances, it is difficult to control the surface temperature of the slab by controlling only the amount of secondary cooling spray water, and to achieve a completely solidified structure free from cracks. In addition to the regulations, we investigated further technology for suppressing cracks at the corners.
Here, the inventors paid attention to the stress load on the slab corner. That is, as shown in FIG. 3, the solidification structure is improved by regulating the secondary cooling conditions, and the degree of cracking at the corners is lighter than that in FIG. 2, so in addition to the secondary cooling conditions If the stress applied to the corner during bending and straightening can be reduced, it was thought that corner cracking could be prevented.

  Therefore, as a result of study by stress calculation, etc., the slab was made chamfered by removing the corners of the four corners of the rectangular cross section perpendicular to the casting direction, thereby reducing the stress load at the corner of the slab. It was found that it can be reduced. And to make the four corners of the slab chamfered, cast using a mold that has a chamfered shape by removing the four corners of the rectangular casting space (the right-angle part) into a right triangle like a rectangular cross-section mold. It is important to do. Hereinafter, a mold having a casting space having such a chamfered shape is also referred to as a chamfer mold.

Here, as for the chamfer mold, for example, Patent Document 3 describes providing corner drop portions at four corners. The technique described in Patent Document 3 aims to normalize the growth of the solidified shell at the corner portion of the slab and prevent defects in the slab due to the solidification delay of the corner portion. Therefore, it is unclear whether the shape of the chamfer described in Patent Document 3 is suitable for preventing the surface crack of the slab as expected in the present invention. That is, in the technique described in Patent Document 3, the solidification of the corner portion of the rectangular cross-section mold is easier to proceed than the other portions in the initial stage of solidification of the steel, and the solidification shell and the rectangular corner portion of the mold are separated by solidification shrinkage. In the case where the air gap that occurred in the mold caused a solidification delay and was likely to become an internal defect, the corner portion of the mold was made into a chamfer shape (chamfered shape), so that the degree of mold cooling at the corner portion other than the corner portion was reduced. This is a state close to mold cooling. Specifically, it gives a chamfer shape in which the four corners of the casting space are evenly removed from each other, but even if such a chamfer mold is used, as shown in FIG. Surface cracking could not be suppressed.
Japanese Patent No. 4864559

  Therefore, as a result of intensive studies to clarify the chamfering shape of the mold suitable for the object of the present invention, it has been found that a new shape regulation different from the conditions described in Patent Document 3 is necessary. Here, when chamfering is performed on the chamfered portion of the chamfer mold by removing the right-angled portions of the corners of the rectangular casting space into a right-angled triangle shape, the right-angled triangle is shown in FIG. The ratio b / a of the length b on the mold short side 12 to the length a on the mold long side 11 side is specified, and the stress calculation is performed on the effect of this ratio b / a on the stress load at the corner of the slab. It was. The calculation results are shown in FIG. 5, organized as an index when the stress in the rectangular mold before chamfering is taken as 100.

  As shown in FIG. 5, it can be seen that the stress load on the corner portion of the slab is first reduced by using the chamfer mold as compared with the rectangular mold. In particular, it can be seen that when the ratio b / a is in the range of 3 to 6, the stress load at the slab corner tends to be reduced. Further, it was also found that the stress load at the slab corner portion becomes smaller as the length a on the mold long side 1 side is smaller.

Under the above-described knowledge, in continuous casting using various molds having the ratio b / a of 1 to 8, the surface temperature of the slab corner is temporarily changed to Ar ₃ point until the slab enters the bending part. When the surface temperature of the slab corner is set to 800 ° C. or higher and the secondary cooling is performed at a temperature of 800 ° C. or higher before the bending portion is entered. When a mold with / a of 3 to 6 was used, surface cracks at the slab corner could be reliably suppressed.
In addition, even if a mold having a ratio b / a of 3 to 6 is used, if the surface temperature of the slab corner portion is not lowered to the Ar ₃ point or lower, it is 800 ° C. or higher before entering the bent portion. In the case where the bending temperature does not reach 800 ° C., a large amount of prior austenite grain boundaries remain in the solidified structure, so that the rate of occurrence of corner cracks cannot be sufficiently reduced.

  Furthermore, the ratio b / a in the mold is preferably more than 4. This is because when the ratio b / a is 4 or less, as shown in FIG. 5, the stress load applied to the corner portion is slightly higher than when b / a is greater than 4 to 6, Because.

  Further, the length a on the long side of the mold is preferably 4 to 6 mm, and the length b on the short side of the mold is preferably 12 to 36 mm. This is because, as shown in FIG. 5, the stress load applied to the corner portion tends to decrease as the length a on the long side decreases, and the length a on the long side is 7 mm. This is because the stress load tends to be slightly larger than the case.

A low alloy steel having a composition shown in Table 1 and having high cracking sensitivity was cast by a vertical bending type continuous casting machine. The Ar ₃ transformation point of this steel is 725 ° C. Casting conditions were a casting thickness of 220 to 300 mm, a casting width of 1400 to 2100 mm, and a casting speed of 0.60 to 2.50 m / min. In continuous casting under these conditions, molds having various chamfered shapes shown in Table 2 were manufactured and used. For comparison, continuous casting using a rectangular mold was performed under the same casting conditions.
The amount of secondary cooling water was changed according to the casting thickness, casting width, and casting speed, but the surface temperature of the slab corner was once lowered below the Ar ₃ transformation point before entering the bending part, and then bent. The heat was reheated before entering the part, adjusted to 800 ° C. or higher, and adjusted using heat transfer analysis so that the bent part passed at 800 ° C. or higher. For comparison, casting was also performed in which the temperature at the corner of the slab did not satisfy the conditions of the present invention.
In addition, the slab temperature at the time of a bending part passage was confirmed by measuring using a thermocouple and a radiation thermometer. In order to facilitate observation of cracks on the surface of the slab, the cast slab is removed by shot blasting to remove the oxide on the surface of the slab, and then a color check (dye penetrant flaw detection test) is performed. The presence or absence of cracks was investigated. And it evaluated by the number of corner crack slabs / the number of investigation slabs x100% as a corner crack occurrence rate. Further, a 30 mm square sample for solidification structure observation was cut out from the corner of the slab, the observation surface was polished, 3% nital corrosion was performed, and the solidification structure was observed with an optical microscope.

  These evaluation results are shown in Table 2. Note that both the inventive example and the comparative example are evaluated for a casting amount of 10 charges (one charge is about 300 tons) at each level.

  Comparative Examples 1 and 2 are examples in which a rectangular mold was used and the slab corner temperature was manufactured under conditions that did not satisfy the present invention. In this case, the crack occurrence rate at the corner was as high as 9.4 to 10.8%. When these solidification structures were observed, the prior austenite grain boundaries as shown in FIG. 2 were clear structures.

  Comparative Examples 3 and 4 use a rectangular mold, and the slab corner temperature is a condition that satisfies the present invention. In this case, the corner crack occurrence rate was 4.7 to 5.2%, which was lower than that of Comparative Examples 1 and 2, but at a level requiring further improvement. As shown in FIG. 3, these solidified structures were structures in which some prior austenite grain boundaries remained.

Comparative Examples 5 to 12 use a chamfer mold, and the slab corner temperature is a condition that does not satisfy the present invention. Also in this case, the corner crack occurrence rate was 5.3-7.3%, which was a level that needed improvement. These solidified structures were also structures with clear prior austenite grain boundaries as shown in FIG.
In Comparative Examples 13 to 15, a chamfer mold is used, and the casting corner temperature is also a condition that satisfies the present invention. However, with respect to the shape of the chamfer portion, the ratio b / a of the length a on the long side and the length b on the short side is a condition that does not satisfy the present invention. Also in this case, the corner crack occurrence rate was 3.8 to 4.5%, which was a level that needed improvement.

  On the other hand, Invention Examples 1 to 8 are conditions in which a chamfer mold is used and the secondary cooling spray is adjusted so that the slab corner temperature satisfies the present invention. As for these, the incidence of corner cracks was as good as 1.4% or less. When these solidified structures were observed, the former austenite grain boundaries remained in a part as shown in FIG. 3 and were the same solidified structures as in Comparative Examples 3 and 4. That is, it was confirmed that corner cracking can be prevented by using a chamfer mold even if the solidified structure is partially incomplete.

  When performing continuous casting under the same conditions as in Example 1, the length a on the long side of the mold is 4 to 7 mm, and the ratio b / a to the length b on the short side of the mold is 3.0 to 6.0. Within the range, a mold in which the length b on the short side of the mold was changed as shown in Table 3 was used. And generation | occurrence | production of the corner part crack was evaluated similarly to the case of Example 1. FIG. The results are also shown in Table 3.

  Among Invention Examples 9 to 32, the occurrence of corner cracks can be completely suppressed when the length a on the long side of the mold is 4 to 6 mm and b / a is more than 4 to 6 . When b / a is 3 to 4, the occurrence of slight corner cracks was observed, but the incidence of these was also 0.6 to 1.4%, which is sufficiently low.

On the other hand, when the length a on the long side of the mold is 7 mm (Invention Examples 33 to 40), some corner cracks are observed even when b / a is more than 4 to 6, and the occurrence rate is 0.6 to 0.9%. Further, the corner crack occurrence rate under the condition of b / a of 3 to 4 was 1.3 to 1.9%. These are also sufficiently low occurrence rates.
That is, it is understood that a preferred example of the present invention is that the mold long side length a is 4 to 6 mm and b / a is 3 to 6, more preferably more than 4 to 6. At that time, the mold short side length b is 12 to 36 mm, more preferably more than 16 mm to 36 mm.
In addition, when the mold long side length a is less than 4 mm, severe processing accuracy is required at the four corners of the mold. Therefore, in actual operation, it is preferably 4 mm or more. Incidentally, the chamfered portion can be formed by, for example, machining a solid copper plate.

As described above, it was confirmed that a high-quality slab having a low corner crack generation rate can be efficiently manufactured by using the chamfer mold of the present invention and controlling the slab corner temperature within an appropriate range.

DESCRIPTION OF SYMBOLS 1 Molten steel 2 Ladle 3 Long nozzle 4 Tundish 5 Immersion nozzle 6 Water-cooled mold 7 Secondary cooling zone 8 Drawing correction zone (bending part)
9 Continuous casting slab 11 Mold long side 12 Mold short side

Claims (3)

In a continuous casting method in which molten steel is charged into a mold and a slab is drawn directly from the mold,
At the four corners of the rectangular space defined by the pair of mold long sides and the pair of mold short sides, the ratio b / a of the length b on the mold short side to the length a on the mold long side is 3.0 or more and 6.0. Using the casting mold, which has the casting space removed in the shape of a right triangle that becomes
Before reaching the bending correction point from directly below the mold, the surface temperature of at least the corner portion of the slab is once lowered to Ar ₃ point or less, and then at least the surface temperature of the corner portion is set to 800 ° C. or higher. A method for continuous casting of steel, wherein the bending correction point is passed at 800 ° C or higher.   The steel continuous casting method according to claim 1, wherein the ratio b / a is more than 4.0.   The continuous casting method for steel according to claim 1 or 2, wherein the length a on the long side of the mold is 4 to 6 mm and the length b on the short side of the mold is 12 to 36 mm.

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