JP2009142838A - Bloom casting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To inhibit the inner cracks and surface cracks of a bloom further by inhibiting the generation of bulging on a narrow face side. <P>SOLUTION: In a bloom casting method, a vertical bending type continuous bloom casting machine 1 is used to cast a bloom having a narrow face size of 250 mm to 450 mm, and containing a carbon content of 0.4 to 1.0% and a chromium content of 0.2 to 1.6%. The method appropriately defines the casing velocity of the continuous bloom casting machine 1, the distance from the casting die 4 to the narrow face support roll 7 on the upper-most stream side, and a surface temperature and a cooling velocity in the narrow surface center at various positions. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a bloom casting method.

Conventionally, many continuous casting methods for casting a bloom have been developed by a vertical bending type bloom continuous casting machine. In bloom casting, the wide surface of the slab pulled out from the mold is supported by the wide surface support roll, and the narrow surface of the slab is supported by the narrow surface support roll, thereby suppressing or preventing the occurrence of bulging. Yes.
The narrow side of the bloom (slab) is thinner than the wide side, and bulging is less likely to occur on the narrow side. The actual situation is that only a few meters are installed from the mold.

However, in recent years, in order to increase productivity, the cross section of the bloom has been increased and the casting speed has been increased. As a result, the occurrence of bulging cannot be ignored even on the narrow side where bulging is difficult to occur. Yes.
In view of this, many techniques have been considered for suppressing the occurrence of bulging on the narrow surface side of the bloom, thereby suppressing internal cracks and surface cracks of the bloom (for example, Patent Document 1).
In Patent Document 1, the narrow surface support roll is arranged in a range up to a position of 2.5 m at the lower end of the mold, and the installation pitch of the narrow surface support roll is larger than the installation pitch of the wide surface support roll, and the lower end of the mold. By setting it to a value smaller than the value defined by the distance from the bulging, the occurrence of bulging on the narrow surface side is suppressed, and the internal crack of the bloom is suppressed.
JP 2006-239769 A

Although the technique of Patent Document 1 can suppress the internal cracking of the bloom by suppressing the occurrence of bulging on the narrow surface side, there are other factors to consider, and there is still room for improvement to be applied to an actual machine. The fact is that is left behind.
Therefore, an object of the present invention is to provide a bloom casting method that can further suppress internal cracking of the bloom by suppressing the occurrence of bulging on the narrow surface side.

In order to achieve the above object, the present invention has taken the following measures.
That is, in a continuous bending type bloom continuous casting machine, the narrow surface size of the bloom to be cast is 250 mm to 450 mm, the carbon component is 0.4% to 1.0%, and the chromium component is 0.2 to 1.6%. A bloom casting method in which the casting speed of the continuous bloom casting machine is in the range of 0.6 to 1.2 m / min and the conditions satisfying the formulas (1) to (5) are satisfied. Is in the point of casting.

FIG. 3 is a diagram showing a cross-sectional view on the narrow surface side of the bloom when bulging occurs. As shown in this figure, in the state where bulging has occurred, compressive strain acts on the solidification interface of the narrow surface central portion 20 so that internal cracks do not occur, but the base interface 21 of the solidified shell (slab) is caused by bulging. Since tensile strain acts, an internal crack 22 is generated in the base portion 21.
Here, in order to suppress internal cracking, when excessive cooling is performed directly under the narrow support roll, the corner portion of the slab is supercooled, and the temperature of the corner portion enters the embrittlement temperature range, Surface cracks will occur on the out side (outside of the bend) of the slab. Therefore, even if it is a case where the internal crack by bulging is suppressed, the surface temperature on the narrow surface side of the slab needs to be set to a predetermined value or more.

From the above, the inventor verified the conditions for preventing internal cracks at the base of the slab and preventing surface cracks from various viewpoints.
As a result, the length from the mold to the narrowest surface support roll on the most upstream side, casting speed, surface temperature and cooling rate at the center of the narrow surface at various positions, bloom narrow surface size, bloom carbon component and chromium component By appropriately defining the above, internal cracks can be suppressed and surface cracks can be prevented.

  According to the present invention, internal cracks and surface cracks of bloom can be further suppressed by suppressing the occurrence of bulging on the narrow surface side.

The bloom casting method of the present invention will be described.
FIG. 1 shows a vertical bending type bloom continuous casting machine, which is an apparatus for performing the bloom casting method of the present invention. The bloom continuous casting machine is not limited to the following apparatus.
As shown in FIG. 1, a continuous casting machine 1 includes a tundish 3 that temporarily stores molten steel 2, a mold 4 that is supplied with the molten steel 2 from the tundish 3, and a slab cast by the mold 4. 5 has a wide surface support roll 6 that supports the wide surface 5, a narrow surface support roll 7 that supports the narrow surface of the slab 5, and a cooling means (cooling nozzle) 8 that cools the slab 5.

The tundish 3 has a bottomed box shape as a whole, and an immersion nozzle 9 is provided at the bottom of the tundish 3. The immersion nozzle 9 can be opened and closed by a slide valve, and the molten steel 2 is injected into and stopped from the mold 4 by opening and closing the immersion nozzle 9.
A plurality of wide surface support rolls 6 are arranged so as to support the wide surface of the slab 5 from the mold 4 toward the downstream side. That is, in the bloom continuous casting machine 1, the wide surface support rolls 6 are arranged in a vertical direction in which the wide surface support rolls 6 are arranged in the vertical direction from the mold 4 toward the downstream side, and the wide surface support rolls 6 are continuously bent from the vertical part 10. A bending portion 11, an arc portion 12 in which the wide surface support roll 6 is arranged in an arc shape from the bending portion 11, and a correction portion 13 in which the wide surface support roll 6 is arranged in an arc from the arc portion 12 are provided.

In other words, the bloom continuous casting machine 1 includes a vertical portion 10 that vertically supports the slab 5 immediately below the mold 4, a bending portion 11 that starts bending the slab 5 following the vertical portion 10, and a bending portion 11. The slab 5 includes an arc portion 12 that makes the slab 5 arc-shaped with a certain curvature, and a correction portion 13 that straightens the slab following the arc portion 12.
A plurality of narrow surface support rolls 7 are arranged so as to support the narrow surface of the slab 5 from the mold 4 toward the downstream side. The wide surface support roll 6 is arranged over a wide range from the vertical part 10 to the correction part 13, but the arrangement area of the narrow surface support roll 7 is narrower than the area of the wide surface support roll 6. For example, it is arranged in an area of at least 2 m from the lower surface of the mold 4 toward the downstream side.

The cooling nozzle 8 performs secondary cooling of the slab 5 that has been primarily cooled by the mold 4 on the downstream side of the mold 4, and a slab is obtained by spraying a cooling medium (for example, water) onto the slab 5. The wide surface side and the narrow surface side of 5 can be cooled, and the amount of water and pressure of the cooling medium can be appropriately set.
In this continuous casting machine 1, the molten steel 2 produced from a converter, secondary refining equipment, etc. is transported to the tundish 3 with a ladle, and the molten steel 2 in the transported ladle is poured into the tundish 3, Casting can be performed by opening the immersion nozzle 9 and charging the molten steel 2 into the mold 4. The slab 5 cast by the mold 4 is drawn downstream while being supported by the wide support roll 6 and the narrow support roll 7, and the surface and the inside thereof are gradually cooled by the cooling nozzle 8.

The bloom casting method of the present invention will be described below.
In casting the bloom, the narrow surface size of the bloom (the width on the narrow surface side of the slab) is 250 mm to 450 mm. The steel grade components of bloom to be cast contain 0.4% to 1.0% carbon and 0.2 to 1.6% chromium. The casting speed is 0.6 to 1.2 m / min.
Furthermore, in casting, the casting is performed under the conditions satisfying the expressions (1) and (2).

As shown in FIG. 1, the formula (1) is the distance (narrow surface support length) from the slab 5 to the narrow surface support roll (sometimes called the most downstream narrow surface roll) 7a located on the most downstream side. L), casting speed Vc, and the central portion of the narrow surface in the width direction of the slab 5 in an area (hereinafter sometimes referred to as a first temperature control area) E1 advanced 1 m from the most downstream narrow surface roll 7a to the downstream side. It shows the relationship with the maximum value Tmax1 of the surface temperature.
When casting the bloom, first, the narrow surface support length L and the casting speed Vc are set in advance so as to satisfy the above-described conditions (L ≧ 2, Vc = 0.6 to 1.2 m / min). In addition, during casting, the amount of water, the pressure, and the dispersion state of the cooling nozzle 8 disposed in the first temperature control area E1 so that the maximum surface temperature Tmax1 in the first temperature control area E1 satisfies the formula (1). (Spray, shower, mist) etc. are controlled.

  Equation (1) is obtained by experiment. Table 1 summarizes the experiment in which the bloom was cast so as to satisfy the formula (1) and the experiment in which the bloom was cast so as not to satisfy the formula (1).

In Experiment 1 to Experiment 21, the steel type of the bloom to be cast was shown as steel type A among the steel types shown in Table 2. The narrow surface support length L was 2.0 m, the length of the mold 4 (vertical height) was 900 mm, and the meniscus was 100 mm from the upper end of the mold 4.
In this experiment, the maximum surface temperature Tmax1 in the first temperature control area E1 was measured with a radiation thermometer. The radiation thermometer is IR-FAINLN manufactured by Chino Co., Ltd., and the measurement range is 300 ° C to 1300 ° C. The measurement error was the larger of ± 5 ° C. or ± 0.5%.
Further, as shown in FIG. 2, in this experiment, the total length of the internal cracks 22 at a depth of 30 mm to 70 mm from the surface in the corner portion of the slab 5 is 20 mm or less (internal crack rating is 20 or less). And those having an internal crack score of 20 or more were defined as defective products (“×” in the table).

From various viewpoints such as prevention of large internal cracks during processing after bloom casting and maintaining quality after product production, the total length of internal cracks 22 (internal crack rating) is required to be 20 or less. Therefore, it was used as a standard for evaluation.
As shown in Table 1, when Bloom is cast so that the maximum surface temperature Tmax1 of the first temperature control area E1 satisfies the formula (1) in the steel types A and L = 2.0 m, the internal crack score is 20 or less. It was possible to suppress the internal crack of the bloom.
On the other hand, in the steel type A, when the maximum surface temperature Tmax1 of the first temperature control area E1 does not satisfy the formula (1), the internal crack score becomes larger than 20, and the required internal crack standard is satisfied. (Internal cracking could not be suppressed).

  Table 3 shows a summary of the results of experiments performed on the steel type B shown in Table 2, in which the steel type is changed with respect to the above experiment.

As shown in Table 3, when bloom was cast so that the maximum surface temperature Tmax1 of the first temperature control area E1 satisfied the formula (1) even in the steel types B and L = 2.0 m, the internal crack score was 20 We were able to:
Table 4 shows the results of experiments conducted on steel type C shown in Table 2 in which the steel type is changed with respect to the above-described experiment.

As shown in Table 4, when the bloom was cast so that the maximum surface temperature Tmax1 of the first temperature control area E1 satisfied the formula (1) even in the steel types C and L = 2.0 m, the internal crack score was 20 We were able to:
According to the experiments in Tables 1 to 4, when casting a bloom containing 0.4% to 1.0% carbon component and 0.2% to 1.6% chromium component, L = 2.0 m, casting speed If it satisfies 0.7-1.1 m / min and Formula (1), an internal crack score can be 20 or less. Even when the casting speed is changed to 1.2 m / min or the steel type D is cast, it is confirmed that the internal crack rating is 20 or less.

Tables 5 to 6 show experimental results verified not only for the narrow surface support length L = 2.0 m but also for L ≧ 2.0 m.
Table 5 summarizes the results of changing the narrow surface support length L when casting the steel type A. In Table 5, casting of steel type A was performed with L = 4.0 m in Experiment 36 to Experiment 56 and L = 7.0 m in Experiment 57 to Experiment 70.

As shown in Table 5, in casting steel type A, as described above, not only L = 2.0 m, but if L ≧ 2.0 m, internal cracks can be prevented.
Table 6 summarizes the results of experiments conducted on other steel types with a narrow surface support length L ≧ 2.0 m. That is, in Table 6, L = 4.0 m is fixed, Experiments 71 to 77 show the results of steel type B, Experiments 78 to 84 show the results of steel type C, and Experiments 85 to 105 show the results of steel type D.

As described above, if the steel types A to D are cast by the experiments shown in Tables 5 to 6, not only L = 2.0 m but also L> 2.0 m (L ≧ 2.0 m) can prevent internal cracking.
Moreover, in casting a bloom, it is made to cast on the conditions which satisfy | fill Formula (3) besides the conditions mentioned above.

As shown in FIG. 1, the expression (3) is an area (hereinafter referred to as a second temperature control area) from 1.5 m immediately below the most downstream narrow surface roll 7a to the bending end position (end of the bending portion 11). It shows the range of the surface temperature T1 at the center of the narrow surface in the width direction of the slab 5 at E2. In other words, Formula (3) shows the surface temperature T1 from the time point of 0.5 m from the most downstream end of the first temperature control area E1 to the end of the bending portion 11.
When casting bloom, during casting, the amount of water in the cooling nozzle 8 disposed in the first temperature control area E1 and the like so that the surface temperature T1 in the second temperature control area E2 satisfies the formula (3) The amount of water of the cooling nozzle 8 arranged in the two temperature control area E2 is controlled.

Equation (3) is obtained by experiment. Table 7 summarizes the experiment in which the bloom was cast so as to satisfy the formula (3) and the experiment in which the bloom was cast so as not to satisfy the formula (3).
In Table 7, L is fixed at 2.0 m, Experiment 106 to Experiment 112 are steel type A (casting speed Vc = 0.7 m / min), and Experiment 113 to Experiment 119 are steel type A (casting speed Vc = 1.1 m / min). min), Experiments 120 to 125 show the results for steel type B (casting speed Vc = 0.7 m / min), and Experiments 126 to 131 show the results for steel type C (casting speed Vc = 0.7 m / min). .

As shown in FIG. 2, in this experiment, the total length of the internal cracks 22 at a depth of 70 mm to 90 mm from the surface at the corner portion of the slab 5 is 20 mm or less (internal crack rating is 20 or less). A product having a non-defective product (“◯” in the table) and an internal crack score of 20 or more was defined as a defective product (“×” in the table).
Further, in the corner portion of the slab 5, the surface crack 23 did not occur, the non-defective product (“◯” in the table), and the surface crack 23 generated, the defective product (“×” in the table). .

As shown in Table 7, when the bloom was cast so that the surface temperature T1 of the second temperature control area E2 satisfies Equation (3) in steel types A to C and L = 2.0 m, the internal crack score was 20 The internal crack of the bloom could be greatly suppressed, and the surface crack of the corner portion could be prevented.
On the other hand, if the surface temperature of the second temperature control area E2 does not satisfy the formula (3) and the surface temperature T1 exceeds 1150 ° C., the internal crack score becomes larger than 20, and the required standard for internal cracks is satisfied. I could not meet. Further, when the surface temperature of the second temperature control area E2 does not satisfy the formula (3) and the surface temperature T1 is less than 1040 ° C., the surface crack of the corner portion occurs.

  Table 8 summarizes the results of changing the narrow surface support length L. In Table 8, L is fixed at 4.0 m, Experiments 132 to 138 are Steel A (casting speed Vc = 0.7 m / min), and Experiments 139 to 145 are Steel A (casting speed Vc = 1.1 m / min). Experiments 146 to 152 are for steel type B (casting speed Vc = 1.1 m / min), experiments 153 to 159 are for steel type C, and experiments 160 to 169 are for steel type D (casting speed Vc = 0.7 m / min). The results are shown.

As shown in Table 8, in steel types A to D, even when L = 2.0 as well as L> 2.0, the bloom was cast so that the surface temperature T1 satisfies the formula (3) In addition to being able to suppress internal cracks, it was possible to prevent surface cracks at the corners.
Further, in casting the bloom, the casting is performed under the condition satisfying the formula (4) in addition to the above-described conditions.

As shown in FIG. 1, the equation (4) represents an area from the correction start position to the correction end position (from the start of the correction unit 13 to the end of the correction unit 13) (hereinafter, may be referred to as a third temperature control area) E3. The range of the surface temperature T2 of the width direction narrow surface center part of the slab 5 is shown.
When casting bloom, during casting, the amount of water, pressure, etc. of the cooling nozzle 8 arranged in the first temperature control area E1 are set so that the surface temperature T2 in the third temperature control area E3 satisfies the formula (4). The amount of water and the pressure of the cooling nozzle 8 disposed in the second temperature control area E2 are controlled.
Equation (4) is obtained by experiment. Table 9 summarizes the experiment in which the bloom was cast so as to satisfy the formula (4) and the experiment in which the bloom was cast so as not to satisfy the formula (4).

In Table 9, L = 2.0 m, Experiment 170 to Experiment 180 are steel type A (casting speed Vc = 0.7 m / min), and Experiment 181 to Experiment 191 are steel type C (casting speed Vc = 0.7 m / min). min). The internal crack score 1 in Table 9 indicates the score in the first temperature control area E1, and the internal crack score 2 indicates the score in the second temperature control area E2.
In the experiment in Table 9, in the corner portion of the slab 5, the total length of the internal cracks 22 at a depth of 90 mm or more from the surface is 20 mm or less (internal crack rating 3 is 20 or less). ("" In the table) and those having an internal crack score of 20 or more were defined as defective products ("X" in the table). Further, in the corner portion (inner corner portion) of the slab 5, the surface crack 23 does not occur, the non-defective product (“◯” in the table), the one in which the surface crack occurs, the defective product (“ × ”).

As shown in Table 9, in steel types A and C, when bloom is cast under the condition that the surface temperature T2 of the third temperature control area E3 satisfies the formula (4), the internal crack rating 3 may be 20 or less. It was possible to greatly suppress the internal crack of the bloom and to prevent the surface crack of the corner portion.
On the other hand, if the surface temperature of the third temperature control area E3 does not satisfy the formula (4) and the surface temperature T2 exceeds 100 ° C., the internal crack rating 3 becomes larger than 20, and the required internal crack standard. Could not meet. Further, when the surface temperature of the third temperature control area E3 does not satisfy the formula (4) and the surface temperature T2 is less than 750 ° C., the surface crack of the corner portion occurs.

For other steel types, casting was performed by changing the narrow surface support length L, casting speed, etc., but satisfying the formula (4) at L ≧ 2.0 m and casting speed 0.6-1.2 m / min. For example, it has been confirmed that internal cracks with a depth of 90 mm or more can be suppressed and surface cracks at the corners can be prevented.
Moreover, in casting a bloom, it is made to cast on the conditions which satisfy | fill Formula (5) besides the conditions mentioned above.

As shown in FIG. 1, the equation (5) represents an area from the bending end position to the correction start position (from the start of the arc portion 12 to the end of the arc portion 12) (hereinafter sometimes referred to as a fourth temperature control area) E4. The average cooling temperature R of the center part of the narrow surface of the slab 5 in the width direction is shown.
When casting the bloom, the water amount and pressure of the cooling nozzle 8 disposed in the first temperature control area E1 so that the average cooling temperature R2 in the fourth temperature control area E4 satisfies the formula (5) during casting. The amount of water and the pressure of the cooling nozzle 8 arranged in the second temperature control area E2 are controlled.
Equation (5) is obtained by experiment. Table 10 summarizes the experiment in which the bloom was cast so as to satisfy the formula (5) and the experiment in which the bloom was cast so as not to satisfy the formula (5).

In Table 10, L is fixed at 2.0 m, Experiment 192 to Experiment 205 is Steel A (casting speed Vc = 0.7 m / min), and Experiments 206 to 219 are Steel C (casting speed Vc = 0.7 m / min). ) Result.
In the experiment in Table 10, in the corner portion of the slab 5, the total number of internal cracks 22 at a depth of 90 mm or more from the surface is 20 mm or less (internal crack rating 3 is 20 or less). “◯”) and those having an internal crack score of 20 or more were defined as defective products (“X” in the table). Moreover, in the corner part (inner corner part) of the slab 5, a non-defective product (table “◯” in the table) in which cracking due to thermal stress has not occurred and a defective product (table in “table”). “×”).

As shown in Table 10, in the steel types A and C, when the bloom was cast under the condition that the cooling rate R of the fourth temperature control area E4 satisfies the formula (5), the internal crack rating 3 may be 20 or less. It was possible to suppress the internal crack of the bloom very much and to prevent the corner portion from being cracked by the thermal stress.
On the other hand, if the cooling rate of the fourth temperature control area E4 does not satisfy the formula (5) and the cooling rate R is less than 2.0 m, the internal crack rating 3 becomes larger than 20 and the required internal cracks. It was not possible to meet the criteria. Further, when the cooling rate R in the fourth temperature control area E4 did not satisfy the formula (5) and the cooling rate R was higher than 28, cracks due to thermal stress occurred in the corner portion.

For other steel types, casting was carried out by changing the narrow surface support length L, casting speed, etc., but L ≧ 2.0 m and casting speed 0.6 to 1.2 m / min can be satisfied. For example, it has been confirmed that internal cracks having a depth of 90 mm or more can be suppressed and cracks due to thermal stress at the corners can be prevented.
As described above, according to the bloom casting method of the present invention, the components of the steel type of the bloom to be cast are assumed to contain carbon components of 0.4% to 1.0% and chromium components of 0.2 to 1.6%, By casting at a casting speed of 0.6 to 1.2 m / min and satisfying the formulas (1) to (5), the internal crack of the bloom can be suppressed, and the corner portion can be cracked. Can be prevented.

  In the above experiment, the bloom size was 600 × 300 mm and 400 × 300 mm. Here, when the relationship between the amount of displacement of the bulging and the width of the narrow surface side of the slab 5 is obtained using a beam bending model, it is as shown in FIG. That is, in the beam bending model, when the roll pitch of the narrow surface support roll 7 is 330 mm and the thickness on the narrow surface side (width on the narrow surface side) of the slab 5 with respect to distortion of the solidification interface base portion (base portion 21) is obtained, FIG. It became like this. In the bending model of the beam, the distortion of the solidification interface base (base 21) when the width on the narrow surface side of the slab 5 is 300 mm was normalized (normalized) as 1.0.

  As shown in FIG. 4, when the distortion when the width of the narrow surface side of the slab 5 is 300 mm is 1.0, it is equivalent to the slab 5 in which the distortion generated by bulging is 0.9. It can be applied as That is, the range of the width of the narrow surface side of the slab 5 where the strain is 0.9 or more is 250 mm to 450 mm. In this range, the width of the narrow surface of the bloom is considered to be almost the same as the above experiment. Similar effects can be obtained in the range of 250 mm to 450 mm. As a result of conducting an experiment similar to that of the present invention on a bloom whose narrow surface size is actually 250 mm or 450 mm, it has also been confirmed that the effects relating to internal cracks and surface cracks are the same as those described above. .

It is a conceptual diagram of a continuous casting machine. It is the figure which showed the inner surface crack and surface crack of the casting_mold | template. It is a figure which shows the relationship between the thickness of a slab, and the solidification interface base distortion. It is a figure which shows sectional drawing by the side of the narrow surface of a bloom at the time of bulging occurring.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Bloom continuous casting machine 2 Molten steel 3 Tundish 4 Mold 5 Slab 6 Wide surface support roll 7 Narrow surface support roll 7a The most downstream narrow surface roll 8 Cooling nozzle 9 Immersion nozzle E1 1st temperature control area E2 2nd temperature control area E3 1st 3 Temperature control area E4 4th temperature control area L Narrow surface support length

Claims (1)

This is a vertical bending type bloom continuous casting machine that casts a bloom with a narrow surface size of 250 mm to 450 mm, a carbon component of 0.4% to 1.0%, and a chromium component of 0.2 to 1.6%. A casting method of
The bloom casting is characterized in that the bloom is cast under conditions satisfying the formulas (1) to (5) at a casting speed of the bloom continuous casting machine in the range of 0.6 to 1.2 m / min. Method.

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