JPH11192539A - Continuous casting method of chromium-containing molten steel with excellent internal defect resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To properly control the complete freezing point of a chromium-containing molten steel in a continuous casting device to prevent voids generated inside a slab, and to segregate due to Zaku, center porosity or molten steel flow. Provided is a continuous casting method of chromium-containing molten steel having excellent internal defect resistance capable of improving the above-mentioned properties. SOLUTION: A chromium-containing molten steel 11 is poured into a casting mold 14 and subjected to primary cooling.
In the continuous casting method of the chromium-containing steel in which the slab 17 is solidified by continuous cooling while being supported inside the slab 17 and continuously pulled out by the pinch roll 18, the solidification completion point 2 of the slab 17
6 is in the curved support segment 16a.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method of chromium-containing molten steel having excellent internal defect resistance, which can prevent the occurrence of internal defects such as center porosity and zigzag in continuous casting of chromium-containing molten steel.

[0002]

2. Description of the Related Art In general, molten steel is poured into a mold to perform primary cooling, and subsequently solidified sequentially by secondary cooling in which cooling water is sprayed from a cooling water nozzle attached to a support device in a continuous casting device. A method of continuously drawing a slab by a pinch roll has been used. In the vicinity of the center of the slab produced by this continuous casting, center porosity and zaku due to insufficient supply of molten steel corresponding to solidification shrinkage, or segregation due to molten steel flow may occur, and a good slab or a good slab There are problems such as a decrease in product yield.
As described above, in order to prevent the center porosity and the backlash in the continuous casting of molten steel, the slab that is undergoing solidification is reduced by an amount corresponding to solidification shrinkage. In particular, as a typical continuous casting method for improving the quality of slabs containing chromium, as shown in JP-A-3-104819, the heating temperature of molten steel in a tundish is set to 20 to 60 ° C. After applying a reduction equivalent to a pressing amount of 1 to 10 times the unsolidified thickness to a range where the unsolidified thickness of the strand is 5 to 30% of the total thickness of the strand (total thickness of the slab), the slab is cooled to a predetermined temperature. Or a method in which the center solid phase ratio of the slab is 0.5 to 0.9 as described in JP-A-4-305350.
A method of reducing the segregation at the center of the slab by reducing the indentation amount by an indentation amount corresponding to 0 to 100% is performed.

[0003]

However, in the continuous casting of chromium-containing molten steel, the above-mentioned Japanese Patent Laid-Open No.
No. 4819, the heating temperature of molten steel is set to 20 to 60.
° C., in a range where the unsolidified thickness is 5 to 30% with respect to the total thickness of the slab (in other words, the central solid phase ratio is 0.7 to 0.95),
The method of applying a reduction of 1 to 10 times the unsolidified thickness can reduce the center porosity and the backlash to some extent, but has the following problems. First, in order to eliminate the center porosity, the backlash, and the like, the portion where the reduction is applied to the slab is in an extremely narrow range, and it is necessary to locally apply a large reduction.
Applying a large reduction in this narrow range causes internal cracks in the slab, and the quality of the slab deteriorates. Further, in order to apply a large amount of reduction in a narrow range, the segment used for the reduction becomes large, and it is difficult to apply a reliable reduction itself, which is poor in practicality. Therefore, a method of reheating the cast slab to perform a large reduction (breakdown method) is required. Furthermore, as in JP-A-4-305350, a reduction of 50 to 100% of the unsolidified thickness is applied to a region where the solid fraction in the center of the slab is 0.5 to 0.9,
In the method of improving the segregation at the center of the slab, the place where the reduction is performed is a range where solidification proceeds rapidly, the effective range for crimping is narrowed, the application of large reduction, the occurrence of internal cracks, the center porosity and the There is a similar problem that it cannot be surely solved. Thus, any of the above-described methods for improving center porosity and Zaku,
This is improved by reducing the slab based on the center solid fraction.

[0006] However, as shown in FIG. 6, the change in the center solid fraction of the cast slab differs greatly between the ordinary steel and the chromium-containing molten steel in the form of solidification. That is, in the case of ordinary steel, solidification progresses slowly as the distance from the meniscus increases, and the range of the center solid fraction (0.2 to 1.0) effective for center porosity or the like by reduction is 2.5 m. Thus, a long distance reduction is possible. This is the liquidus temperature (T
LL) and the solidus temperature (TSL) are as large as 35 to 45 ° C., and this is due to the solidification characteristic that solidification becomes slow. Porosity can be eliminated. On the other hand, in the chromium-containing molten steel described above, the liquidus temperature (TLL) and the solidus temperature (TSL)
L) has a very narrow temperature difference of about 15 ° C., so that the center solid phase ratio (0.2
To 1.0) is 0.57 m, which is a very narrow range of reduction. When the reduction is performed in this narrow range, the reduction using the center solid fraction depending on the liquidus temperature (TLL) and the solidus temperature (TSL) as an index may cause the above-described large reduction, generation of internal cracks, uncompression, etc. It is impossible to eliminate Zaku and center porosity.

The present invention has been made in view of such circumstances, and by appropriately controlling the solidification of a chromium-containing molten steel in a continuous casting apparatus, eliminating the causes of the generation of center porosity and sagging, the casting accompanying the reduction of the casting. It is an object of the present invention to provide a continuous casting method of chromium-containing molten steel excellent in internal defect resistance capable of preventing internal cracking of a piece and preventing occurrence of segregation due to center porosity, Zaku or flow of molten steel.

[0006]

According to the present invention, there is provided a semiconductor device comprising:
The continuous casting method of chromium-containing molten steel having excellent internal defect resistance according to the description is performed by pouring chromium-containing molten steel into a mold and performing primary cooling, and then performing secondary cooling while continuously supporting the slab in the support segment. In the continuous casting method of chromium-containing molten steel, in which a slab is solidified and continuously drawn by a pinch roll, a solidification completion point of the slab is set in a curved support segment. Here, the solidification completion point of the slab is a point at which the inside of the slab is completely solidified by primary cooling in the mold and secondary cooling in the support segment. Further, the curved support segment in the support segment refers to all the support segments in which the support segment of the slab in the continuous casting apparatus is curved at a predetermined curvature after the mold. By providing the solidification completion point of the slab in this curved support segment, the angle of the unsolidified portion formed before the solidification completion point can be increased, and the containment of the chromium-containing molten steel in the solidification process is eliminated. be able to.

According to a second aspect of the present invention, there is provided a method for continuously casting chromium-containing molten steel having excellent internal defect resistance, wherein the casting speed of the chromium-containing molten steel is excellent. To 0.2 to 1.4 m / min. When the casting speed is higher than 1.4 m / min, the angle of the unsolidified portion formed before the solidification completion point becomes small because the solidification completion point of the slab cannot be maintained in the curved support segment. Chromium-containing molten steel is confined, and defects such as center porosity and zaku occur.
On the other hand, when it is slower than 0.2 m / min, the surface of the slab is supercooled, and surface defects are generated, or cracks are generated when the slab is corrected horizontally from the curvature.

[0008] A continuous casting method of chromium-containing molten steel excellent in internal defect resistance according to claim 3 is a method for continuously casting chromium-containing molten steel excellent in internal defect resistance according to claim 1 or 2. The temperature of the chromium-containing molten steel is
The temperature is 10 to 20 ° C. higher than the liquidus temperature of the chromium-containing molten steel. Here, if the temperature added to the liquidus temperature of the chromium-containing molten steel poured into the mold is lower than 10 ° C., the base metal adheres to the nozzles, stoppers and the like used for casting, and the nozzles are clogged and the casting is hindered. On the other hand, if the temperature is higher than 20 ° C., it becomes difficult to maintain the solidification completion point within the curved portion, and the angle of the unsolidified portion formed before the solidification completion point becomes small, so that the containment of the chromium-containing molten steel becomes difficult. It is easy to occur.

[0009]

Next, a continuous casting method of chromium-containing molten steel having excellent internal defect resistance according to an embodiment of the present invention will be described with reference to the accompanying drawings to provide an understanding of the present invention. FIG. 1 is an overall conceptual diagram of a continuous casting apparatus to which a continuous casting method of chromium-containing molten steel having excellent internal defect resistance according to an embodiment of the present invention is applied, and FIG. 2 is an unsolidified steel formed before a solidification completion point. FIG. 3 is a schematic diagram showing the crater angle of the portion, FIG. 3 is a diagram showing the casting speed and the distance from the meniscus to the solidification completion point, FIG. 4 is a diagram showing the distance from the meniscus to the solidification completion point and the crater angle at that time, and FIG. The casting speed and the center porosity maximum minor diameter (mm) of Examples and Comparative Examples of the continuous casting method of chromium-containing molten steel having excellent internal defect resistance according to the invention.
FIG. 7 is a diagram showing a relationship between the score and the center porosity score. As shown in FIG. 1, a continuous casting apparatus 10 uses a submerged nozzle 13 provided at the bottom of a tundish 12 to mold a mold 14.
And a support segment 16 for guiding and supporting a slab 17 solidified by cooling by pouring molten steel 11 containing chromium. And the chromium-containing molten steel 11 poured into the mold 14
The solidified shell 15 is formed by primary cooling of the mold 14. After the primary cooling, the curved support segment 16a and the horizontal support segment 16b of the support segment 16 are further cooled.
Cooling water is sprayed from a cooling water nozzle (not shown) attached to the slab to perform secondary cooling, and the cast slab 1 on which the solidified shell 15 has grown.
7 is pulled out by a pinch roll 18. The level (meniscus) 19 of the molten metal level formed in the mold 14 is measured by a meniscus position detector 20 using an eddy current sensor or a laser beam, input to the arithmetic and control unit 21, and recorded as a change with time. .
When the meniscus 19 fluctuates, a command for adjusting the opening degree of the nozzle (not shown) is output from the arithmetic and control unit 21 so that the supply amount of the chromium-containing molten steel 11 from the immersion nozzle 13 can be adjusted. . Furthermore, the drawing speed (casting speed) of the slab 17 is input to the arithmetic and control unit 21 by converting the rotation speed of the pinch roll 18 into a casting speed, and is recorded by the arithmetic and control unit 21 for each elapse. . Further, the arithmetic and control unit 21 receives data obtained in advance from the relationship between operating conditions such as steel type, molten steel temperature, cooling conditions, casting speed and the position of the solidification completion point 26 under those conditions. When the solidification completion point 26 is specified at an arbitrary position in the segment 16a, the arithmetic and control unit 21 can determine operating conditions such as casting speed and cooling conditions. For example, when the position of an arbitrary solidification completion point 26 is designated, the rotation speed of the pinch roll 18 is controlled to be set to a predetermined casting speed, and the solidification completion point 26 is held in the curved support segment 16a. Casting is performed.
Further, the cast piece 17 solidified by the curved support segment 16a and the horizontal support segment 16b can be reduced by the plurality of reduction segments 22. This rolling segment 22 is connected to a hydraulic cylinder (not shown)
A plurality of roll supporting frames 23 supported so as to be freely pressed
And a plurality of convex rolls 24 for rolling down the slab 17 are provided. The slab 17 pressed down by the convex roll 24 is continuously pulled out at a predetermined speed by a pinch roll 18 and is conveyed to a subsequent process.

Next, the operation when the continuous casting apparatus 10 is applied to the method for continuously casting chromium-containing molten steel having excellent internal defect resistance according to one embodiment of the present invention will be described. A chromium-containing molten steel 11 containing 0.19% by weight of carbon and 10 to 20% by weight of chromium refined by a converter and a vacuum refining furnace (not shown) was used. At the end of refining, the chromium-containing molten steel 11 has a temperature of 1515 to 1
Adjusted to 535 ° C., the tundish 12 was immersed through the immersion nozzle 13 to form a square mold 1
4 was poured. After pouring, primary cooling by the mold 14 and secondary cooling by the curved support segment 16a and the horizontal support segment 16b were performed to promote solidification. This secondary cooling is performed by a cooling water nozzle (not shown) attached to the curved support segment 16a and the horizontal support segment 16b.
And water was sprinkled on the slab 17. The amount of water sprayed is 40% of the total water volume (705 liters / minute) at the upper part of the curved support segment 16a, the lower part of the curved support segment 16a and the horizontal support segment 16b are combined to achieve a cooling water amount of 60%. Casting was performed at a casting speed of 2 to 1.4 m / min. As shown in FIG. 2, the cross section of the cast piece 17 passing through the curved support segment 16a is formed with an unsolidified portion 25 and a solidified shell 15 formed therein.
Starting from 6, a crater angle θ is also formed. Further, as the unsolidified portion 25 progresses (time elapses) in the pulling-out direction (arrow in FIG. 2), solidification progresses to a solidification completion point 26, and the entire cast piece 17 solidifies.

FIG. 3 shows a change in the casting speed (m / min) and the position of the solidification completion point 26 when the molten steel temperature and cooling conditions during casting are kept constant. FIG. And the crater angle θ. The solidification completion point 26 has a longer distance from the meniscus 19 because the solidification of the slab 17 is delayed as the casting speed increases, while the crater angle θ is a distance from the meniscus 19 to the solidification completion point 26. The opening angle decreases as the length increases. When the casting speed is higher than 1.4 m / min, the position of the solidification completion point 26 is shifted to the meniscus 19.
And more than 20 m (the distance to the final end of the curved support segment 16a), so that
The solidification completion point 26 of the slab 17 cannot be maintained, and the crater angle θ of the unsolidified portion 25 is also reduced to less than 2 ° 40 ′. As a result, the chromium-containing steel 11 is confined, solidification and shrinkage of the chromium-containing steel 11 forms a cavity, and causes defects such as center porosity and zaku. The reason for this is that the unsolidified portion 25 is formed while shrinking with the progress of solidification, and columnar crystals (dendrites) grow from the top and bottom of the solidification shell 15 toward the center. When the casting speed is higher than 1.4 (m / min), or when the crater angle θ is smaller than 2 ° 40 ′, the dendrite that grows inward from above and below the solidified shell 15 is formed. This is because the frequency at which the upper end and the lower end are connected to each other increases, and the frequency of the containment of the chromium-containing molten steel 11 inside the cast piece 17 also increases. This contained chromium-containing molten steel 11
Is shrunk as the solidification progresses, and a cavity is formed inside the slab 17, resulting in defects such as center porosity and zigzag. If the casting speed is slower than 0.2, the surface of the slab 17 is rapidly cooled to cause cracks, resulting in surface defects, or when correcting the slab 17 horizontally from the curvature, The elongation of the piece 17 is reduced and cracks occur.
For these reasons, the casting speed is more preferably set to 0.3 to 1.3 m / min. Note that the crater angle θ is set so that the meniscus 1 is not solidified at the position of the meniscus 19.
This is a value that is の of the entire angle of the unsolidified portion 25 when the region from 9 to the solidification completion point 26 is approximated to a straight line.

The thickness of the unsolidified portion 25 and the solidification completion point 26 are determined according to the respective casting conditions. From the erosion state of the cross-section stud, the unsolidified portion 25
Is determined, input to the arithmetic and control unit 21, and recorded. Alternatively, the thickness of the unsolidified portion 25 can be determined by a general heat transfer calculation using the type of steel, thermal conductivity under cooling conditions, specific heat, and the like. Further, a plurality of convex rolls 24 of the reduction segment 22 are added to the slab 17 cast at the above-described casting speed.
In this case, the reduction may be performed with a reduction amount of 1 to 5% of the total thickness of the slab 17.
When the rolling reduction is more than 5%, the pressure bonding to the slab 17 becomes excessive and the slab 17 is cracked. Therefore, the rolling reduction is more preferably 1 to 3%.

Next, an example of a method for continuously casting chromium-containing molten steel having excellent internal defect resistance according to an embodiment of the present invention will be described. As the chromium-containing molten steel 11, carbon 0.
It contains 19% by weight, 0.60% by weight of manganese, and 13% by weight of chromium.
The temperature was adjusted to 1520 ° C., which is higher by 5 ° C., and the width was 220 mm and the thickness was 2 from the tundish 12 through the immersion nozzle 13.
Pouring was performed on a 20 mm square mold 14. After pouring, the primary cooling by the casting mold 14 and the upper part of the curved support segment 16a, 40% of the total water volume (705 liter / min), the lower part of the curved support segment 16a and the remaining part of the horizontal support segment 16b Secondary cooling was performed by spraying 60% of cooling water to promote solidification. The casting speed is 1.15
The solidification completion point 26 is set at the meniscus 1 at ~ 1.4 (m / min).
Controlled within a curved support segment 16a of 9 to approximately 15-20 m.

FIG. 5 shows the center porosity maximum minor axis (mm) and the center porosity rating. When the casting speed is set to 1.15 to 1.4 (m / min) and the solidification completion point 26 is kept within about 15 to 20 m from the meniscus 19 (Example), the maximum minor diameter of the center porosity is set to 4 mm or less ( It is possible to reduce the occurrence of Zaku and center porosity, and the center porosity score evaluated to 4 levels based on the size of the center porosity maximum minor axis is also 2 or less. In this case, the internal defects were few and good. On the other hand, the casting speed was 1.44 (m / min) or more, and the
When 6 is outside the curved support segment 16a (comparative example), the containment portion of the chromium-containing molten steel 11 is formed, and the maximum minor axis diameter of the center porosity is increased to 7 to 15 mm (indicated by ● in the figure), and the center porosity is increased. The rating was 3 or more, and the pipes were turned into waste during pipe making, and the yield was reduced, which was a bad state.

As described above, the continuous casting method of chromium-containing molten steel having excellent internal defect resistance according to one embodiment of the present invention has been described. However, the present invention includes other than this without departing from the gist of the present invention. . In addition, in order to make the solidification completion point 26 exist in the curved support segment 16a, the cooling is performed by increasing the total cooling water amount or increasing the ratio of the cooling water supplied to the curved support segment 16a to more than 40% other than the casting speed. It is also possible to strengthen and solidify quickly.

[0016]

According to the method for continuously casting chromium-containing molten steel excellent in internal defect resistance according to claims 1 to 3, since the solidification completion point of the slab is in the curved support segment, it is formed immediately before the solidification completion point. The crater angle of the unsolidified portion increases, and the chromium-containing molten steel is not contained, thereby preventing the formation of a cavity inside the slab. By suppressing the cavities generated inside the cast slab, center porosity, zigzag, segregation and the like inside the cast slab can be prevented, so that a pipe material and other steel materials having excellent internal quality can be obtained.

In particular, according to the continuous casting method for chromium-containing molten steel excellent in internal defect resistance according to claim 2, the casting speed of the slab is set to 0.2 to 1.4 m / min. In the curved support segment to increase the crater angle of the unsolidified portion, to prevent defects such as center porosity and Zaku, and to reduce surface defects or curved portions caused by overcooling of the slab surface. Can be prevented from cracking at the time of straightening to the horizontal part.

According to a third aspect of the present invention, there is provided the continuous casting method of chromium-containing molten steel excellent in internal defect resistance, wherein the temperature of the chromium-containing molten steel poured into a mold is higher than the liquidus temperature of the chromium-containing molten steel by 10 to 20 ° C. Because it has been prevented, nozzle clogging during continuous casting is prevented, the solidification completion point is held in the curved support segment, and the crater angle of the unsolidified part can be easily increased,
Defects such as center porosity and Zaku can be stably prevented.

[Brief description of the drawings]

FIG. 1 is an overall conceptual diagram of a continuous casting apparatus to which a continuous casting method for chromium-containing molten steel having excellent internal defect resistance according to an embodiment of the present invention is applied.

FIG. 2 is a schematic diagram illustrating a crater angle of an unsolidified portion formed immediately before a solidification completion point.

FIG. 3 is a diagram showing a casting speed and a distance from a meniscus to a solidification completion point.

FIG. 4 is a diagram showing a distance from a meniscus to a solidification completion point and a crater angle at that time.

FIG. 5 is a view showing the relationship between the casting speed, the maximum minor diameter (mm) of center porosity, and the center porosity rating in Examples and Comparative Examples of the continuous casting method of chromium-containing molten steel having excellent internal defect resistance according to the present invention.

FIG. 6 is a diagram showing a comparison of the center solid fraction of conventional ordinary steel and chromium-containing molten steel.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 Continuous casting apparatus 11 Chromium-containing molten steel 12 Tundish 13 Immersion nozzle 14 Mold 15 Solidification shell 16 Support segment 16a Curved support segment 16b Horizontal support segment 17 Cast piece 18 Pinch roll 19 Metal surface level (meniscus) 20 Meniscus position detector 21 Operation control Apparatus 22 Roll-down segment 23 Roll support frame 24 Convex roll 25 Unsolidified part 26 Solidification complete point

Claims (3)

[Claims] 1. A chromium-containing molten steel is poured into a mold to perform primary cooling, and then secondary cooling is performed while continuously supporting the slab in a support segment to solidify the slab and continuously use a pinch roll. A method for continuously casting chromium-containing molten steel, comprising: setting a solidification completion point of the slab in a curved support segment; 2. The casting speed of the chromium-containing molten steel is set to 0.2.
The method for continuous casting of chromium-containing molten steel excellent in internal defect resistance according to claim 1, characterized in that the pressure is set to ~ 1.4 m / min. 3. The temperature of the chromium-containing molten steel poured into the mold is 10 to 2 times higher than the liquidus temperature of the chromium-containing molten steel.
3. The method for continuous casting of chromium-containing molten steel excellent in internal defect resistance according to claim 1, wherein the temperature is 0 ° C. higher.