JP2023141391A - Continuous casting method for steal - Google Patents

Abstract

To provide a continuous casting method for steal that can suppress surface cracking of a cast piece while reducing center segregation and porosity.SOLUTION: A continuous casting machine (1) is used in a continuous casting method for steal. The continuous casting method comprises an application step, a water amount adjustment step, and a pressing step. In the application step, a magnetic field with a flux density of 1000 Gauss or higher is applied to the molten steel (M) in a casting mold (4) by an electromagnetic brake (4a). In the water amount adjustment step, the amount of water injected from a second nozzle (6B) is adjusted to 1.5 times or more as much as the amount of water injected from the first nozzle (6A) from the time the thickness of the solidified shell (S) of the cast piece (10) reaches 30 mm until it reaches 70 mm. In the pressing process, a cast piece (10) is lightly pressed in the thickness direction perpendicular to the width direction with the light pressing roll (7) at a pressing rate of 0.5 mm/min or more and 1.3 mm/min or less from when the central solidus of the cast piece (10) reaches 0.1 until it reaches 1.0.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to a method for continuous casting of steel.

During continuous steel casting, defects such as center segregation and porosity occur in slabs. The slab obtained by continuous casting is rolled into a product. In recent years, products have become thicker and stronger, and as a result, demands on the internal quality of slabs have further increased. Therefore, it is required to further reduce center segregation and porosity at the center of the slab in the thickness direction. In order to improve the internal quality caused by these defects, the slab is usually lightly reduced in the thickness direction in a continuous casting machine.

If the thickness of the solidified shell of a slab in the middle of solidification is non-uniform across the width, there will be regions where the completion of solidification is relatively delayed. In particular, center segregation and porosity tend to increase in regions where the completion of solidification is delayed.

Usually, in continuous casting, a submerged nozzle having two to four discharge holes is used, and hot molten steel is supplied from the discharge holes within the mold. This high-temperature molten steel is discharged toward the widthwise ends of the solidified shell. Therefore, the vicinity of the ends in the width direction of the slab tends to reach a high temperature, and the completion of solidification tends to be delayed.

Further, a continuous casting machine used in continuous casting includes a support roll that guides the slab to the downstream side in the casting direction, and a nozzle that injects secondary cooling water to the slab. Generally, the support roll is divided into two to four roll body parts in the width direction of the slab in order to distribute the load due to static pressure or rolling of the molten steel. The roll body parts are connected to each other by a bearing part. The secondary cooling water injected from the nozzle toward the slab flows down the surface of the slab toward the downstream side in the casting direction, and accumulates at the contact portion between the roll body and the slab. The secondary cooling water accumulated in the contact portion is discharged from the gap between the bearing portion of the support roll and the slab, and is also discharged from the widthwise end portion of the slab. The part of the slab that comes into contact with the support roll is cooled by the secondary cooling water collected in the contact part and the support roll that comes into contact with the slab. On the other hand, secondary cooling water does not accumulate in the area of the slab corresponding to the bearing part of the support roll, and this area does not come into contact with the support roll. Therefore, the region of the slab corresponding to the bearing portion of the support roll is less likely to be cooled than the contact portion with the support roll, and completion of solidification is likely to be delayed.

As explained above, due to the flow of molten steel in the mold and non-uniform secondary cooling of the slab, there are regions in the slab that are in the middle of solidification where the completion of solidification is likely to be delayed. As a result, the thickness of the solidified shell of the slab during solidification becomes non-uniform across the width. In such a case, even if the slab is lightly rolled down in the thickness direction downstream in the casting direction, center segregation and porosity will not be reduced in areas where completion of solidification is delayed, making it difficult to improve internal quality. Therefore, it is required to make the thickness of the solidified shell of the slab uniform in the width direction.

In response to such a demand, for example, Patent Documents 1 and 2 disclose a technique for making the thickness of the solidified shell of a slab uniform by controlling the amount of secondary cooling water supplied to the slab across the width. There is. Patent Document 1 discloses that the solidification profile in the width direction of the slab (change in the thickness of the solidified shell in the width direction of the slab) is determined in advance, and the amount of secondary cooling water in the width direction of the slab is determined based on this profile. A technique for controlling is disclosed. Patent Document 2 discloses a technology in which a device that can detect the solidification completion position of a slab online is installed in a continuous casting machine, and the amount of secondary cooling water in the width direction of the slab is controlled based on the information. .

Patent No. 6561822 Japanese Patent Application Publication No. 2008-238256

In recent years, continuous casting has tended to add more alloying components to molten steel than ever before in order to improve product performance. When the alloy components increase, cracks are more likely to occur on the surface of the slab. If cracks occur on the surface of the slab, the cost of cleaning the surface increases and the yield decreases. Therefore, it is required not only to reduce center segregation and porosity, but also to suppress surface cracking of slabs.

The technique of Patent Document 1 is based on the premise that negative segregation is formed at the center of the thickness. In order to form negative segregation at the center of the thickness, it is necessary to greatly reduce the slab in the middle of solidification using the unsolidified large reduction method. In the unsolidified large reduction method, a large reduction greater than the amount of solidification shrinkage is applied to the slab in the middle of solidification, and concentrated molten steel is forcibly discharged. This unsolidified large reduction method is a different technology from the light reduction method. In the light reduction method, a slab in the middle of solidification is reduced by an amount of reduction corresponding to the amount of solidification shrinkage to alleviate positive segregation at the center of thickness. Further, the technique of Patent Document 2 requires a sophisticated device that can detect the completion of solidification of the slab on-line, which is disadvantageous in terms of cost effectiveness because equipment development, installation, and maintenance costs are high.

Furthermore, Patent Documents 1 and 2 do not give any consideration to suppressing surface cracks in the slab. As described in Patent Documents 1 and 2, when attempting to make the thickness of the solidified shell of a slab uniform in the width direction by simply controlling the amount of secondary cooling water across the width of the slab, The surface temperature of the ends in the width direction of the cast slab may decrease, and cracks may occur on the surface of the cast slab.

An object of the present disclosure is to provide a continuous steel casting method that can suppress surface cracking of slabs while reducing center segregation and porosity.

A continuous casting method for steel according to the present disclosure uses a continuous casting machine. The continuous casting machine includes a mold, a support roll, a first nozzle, a second nozzle, and a light reduction roll. The mold includes an electromagnetic brake. The support roll is installed downstream of the mold in the casting direction, and is divided into a plurality of roll body parts in the width direction of the slab. The roll body parts are connected to each other by a bearing part. The first nozzle injects secondary cooling water to a region of the slab corresponding to the roll body. The second nozzle injects secondary cooling water to a region of the slab corresponding to the bearing portion. The light reduction roll is installed downstream of the support roll in the casting direction. The continuous casting method includes an impression process, a water amount adjustment process, and a rolling process. In the application process, a magnetic field with a magnetic flux density of 1000 Gauss or more is applied to the molten steel in the mold using an electromagnetic brake. In the water amount adjustment step, the amount of water injected from the second nozzle is set to be at least 1.5 times the amount of water injected from the first nozzle until the thickness of the solidified shell of the slab reaches 30 mm and reaches 70 mm. In the rolling process, the slab is rolled at a rolling speed of 0.5 mm/min or more and 1.3 mm/min with a light rolling roll from when the solid fraction at the center of the slab reaches 0.1 until the solid fraction at the center reaches 1.0. Lightly reduce the slab in the thickness direction perpendicular to the width direction at the following reduction speed.

According to the continuous steel casting method according to the present disclosure, surface cracking of the slab can be suppressed while reducing center segregation and porosity.

FIG. 1 is a schematic diagram of a continuous casting machine used in the continuous casting method according to the first embodiment. FIG. 2 is a cross-sectional view of the continuous casting machine of the first embodiment as viewed along the casting direction. FIG. 3 is a cross-sectional view of the continuous casting machine of the second embodiment as viewed along the casting direction.

In the continuous steel casting method according to this embodiment, a continuous casting machine is used. The continuous casting machine includes a mold, a support roll, a first nozzle, a second nozzle, and a light reduction roll. The mold includes an electromagnetic brake. The support roll is installed downstream of the mold in the casting direction, and is divided into a plurality of roll body parts in the width direction of the slab. The roll body parts are connected to each other by a bearing part. The first nozzle injects secondary cooling water to a region of the slab corresponding to the roll body. The second nozzle injects secondary cooling water to a region of the slab corresponding to the bearing portion. The light reduction roll is installed downstream of the support roll in the casting direction. The continuous casting method includes an impression process, a water amount adjustment process, and a rolling process. In the application process, a magnetic field with a magnetic flux density of 1000 Gauss or more is applied to the molten steel in the mold using an electromagnetic brake. In the water amount adjustment step, the amount of water injected from the second nozzle is set to be at least 1.5 times the amount of water injected from the first nozzle until the thickness of the solidified shell of the slab reaches 30 mm and reaches 70 mm. In the rolling process, the slab is rolled at a rolling speed of 0.5 mm/min or more and 1.3 mm/min with a light rolling roll from when the solid fraction at the center of the slab reaches 0.1 until the solid fraction at the center reaches 1.0. Light reduction is performed in the thickness direction perpendicular to the width direction of the slab at the following reduction speed (first configuration).

In the continuous casting method according to the first configuration, in the application step, a magnetic field having a magnetic flux density of 1000 Gauss or more is applied to the molten steel in the mold using an electromagnetic brake. Thereby, the flow of molten steel discharged from the discharge hole of the immersion nozzle toward the widthwise end of the solidified shell within the mold can be weakened. Further, in the continuous casting method according to the first configuration, in the water amount adjustment step, the amount of water injected to the area of the slab corresponding to the roll body of the support roll is adjusted to the area corresponding to the bearing part of the support roll of the slab. The amount of water should be at least 1.5 times the amount of water injected into the area. Therefore, it is possible to sufficiently cool the area of the slab corresponding to the bearing portion of the support roll where completion of solidification is likely to be delayed. In this way, by controlling both the flow of molten steel in the mold and the amount of secondary cooling water, the thickness of the solidified shell of the slab can be made uniform in the width direction. By lightly rolling down this slab in the thickness direction in the rolling process, center segregation and porosity can be reduced.

In the continuous casting method according to the first configuration, the thickness of the solidified shell of the slab is controlled not only by controlling the amount of secondary cooling water in the width direction but also by controlling the flow of molten steel in the mold using an electromagnetic brake. Make it uniform in the width direction. In this case, the surface temperature of the slab is less likely to drop compared to the case where the thickness of the solidified shell of the slab is made uniform in the width direction only by controlling the amount of secondary cooling water. Thereby, it is possible to suppress the occurrence of cracks on the surface of the slab.

In the continuous casting method of the first configuration, the continuous casting machine may further include a third nozzle that injects secondary cooling water to the ends in the width direction of the slab. In that case, in the water amount adjustment step, the amount of water injected from the third nozzle is set to be 1.5 times or more the amount of water injected from the first nozzle (second configuration).

According to the continuous casting method according to the second configuration, it is possible to sufficiently cool the widthwise ends of the slab, where completion of solidification is likely to be delayed, as well as the area of the slab corresponding to the bearing portion of the support roll. can. Thereby, center segregation and porosity can be further reduced.

Embodiments of the present disclosure will be described below with reference to the drawings. In each figure, the same or equivalent components are designated by the same reference numerals, and the same description will not be repeated.

[First embodiment]
[Continuous casting machine]
FIG. 1 is a schematic diagram of a continuous casting machine 1 used in the continuous casting method according to the present embodiment. In the continuous casting machine 1, slabs 10 are manufactured. The continuous casting machine 1 includes a tundish 2, a mold 4, a plurality of support rolls 5, and a plurality of light reduction rolls 7.

Molten steel M is supplied to the tundish 2 from a ladle (not shown). Molten steel M in the tundish 2 is supplied to the mold 4 via the immersion nozzle 3. Molten steel M in the mold 4 is cooled by the mold 4. The mold 4 includes an electromagnetic brake 4a. The electromagnetic brake 4a is, for example, an electromagnet. For example, electromagnetic brakes 4a are arranged on both outer sides of the mold 4 in the thickness direction of the slab 10. Here, the thickness direction is a direction perpendicular to the width direction of the slab 10 and the casting direction.

A plurality of secondary cooling nozzles (not shown in FIG. 1) and a plurality of support rolls 5 are arranged downstream of the mold 4 in the casting direction. After being cooled in the mold 4, the molten steel M is further cooled by being injected with secondary cooling water from a secondary cooling nozzle. As a result, a solidified shell S is formed. Here, the slab 10 in the middle of solidification includes a solidified shell S (solid phase ratio is 1.0) and molten steel M in an unsolidified state (solid phase ratio is less than 1.0). A slab 10 that is in the middle of solidification and contains molten steel M in an unsolidified state is guided downstream in the casting direction by a plurality of support rolls 5 . In the process, the unsolidified molten steel M gradually decreases, and a completely solidified slab 10 is formed. For example, the nozzles for secondary cooling are arranged alternately with a plurality of support rolls 5 in the casting direction.

FIG. 2 is a cross-sectional view of the continuous casting machine 1 of this embodiment as viewed along the casting direction. In FIG. 2, the lower part of the mold 4 is shown. Referring to FIG. 2, the support roll 5 is divided into a plurality of roll body parts 5a in the width direction of the slab 10. The roll body parts 5a are connected to each other by a bearing part 5b. The bearing portion 5b rotatably supports the roll body portion 5a. The dimension of the bearing portion 5b in the width direction is, for example, 150 to 250 mm. The arrangement of the roll main body portion 5a and the bearing portion 5b is typically symmetrical in the width direction of the slab 10.

The slab 10 contacts the roll main body 5a of the support roll 5, and is guided downstream in the casting direction by the roll main body 5a. A region in the width direction of the slab 10 that contacts the support roll 5 (roll body portion 5a) is referred to as a contact portion 10a. The bearing portion 5b does not come into contact with the slab 10 and is spaced apart from the slab 10 in the thickness direction of the slab 10. A region in the width direction of the slab 10 that is not in contact with the support roll 5 is referred to as a non-contact portion 10b. Here, the contact portion 10a is a region of the slab 10 corresponding to the roll body portion 5a, and the non-contact portion 10b is a region of the slab 10 corresponding to the bearing portion 5b. The contact portion 10a and the non-contact portion 10b include not only the region of the slab 10 where the support rolls 5 are arranged, but also the region between the two support rolls 5 in the casting direction.

The plurality of secondary cooling nozzles are arranged at intervals in the width direction. In a typical example, a plurality of secondary cooling nozzles are arranged between adjacent support rolls 5. The plurality of secondary cooling nozzles include a first nozzle 6A and a second nozzle 6B. The first nozzle 6A and the second nozzle 6B are arranged along the width direction of the slab 10. The first nozzle 6A injects secondary cooling water to the contact portion 10a of the slab 10. The second nozzle 6B injects secondary cooling water to the non-contact portion 10b of the slab 10. The amount of cooling water injected from each of the first nozzle 6A and the second nozzle 6B is individually controlled.

A plurality of light reduction rolls 7 are arranged downstream of the plurality of support rolls 5 in the casting direction. The light reduction rolls 7 each form a pair and lightly reduce the slab 10. More specifically, a series of light reduction rolls 7 constitute a light reduction band 71 . The light rolling band 71 is divided into a plurality of segments, and a plurality of light rolling rolls 7 are provided for each segment. The amount of reduction of the light reduction roll 7 is controlled for each segment. The plurality of light rolling rolls 7 are arranged at approximately equal intervals between the inlet 71i and the outlet 71o of the light rolling zone 71. Here, in the light reduction zone 71, the position of the inlet 71i coincides with the position of the light reduction roll 7 disposed at the most upstream position in the casting direction, and the position of the outlet 71o coincides with the position of the light reduction roll 7 disposed at the most downstream position in the casting direction. This corresponds to the position of the reduction roll 7.

[Continuous casting method]
The continuous casting method for steel according to the present embodiment includes an impression process, a water amount adjustment process, and a rolling process. The slab 10 obtained by this continuous casting method becomes a material for products such as steel plates.

In the application process, a magnetic field is applied to the molten steel M in the mold 4 by the electromagnetic brake 4a. As a result, the molten steel M discharged from the discharge hole of the immersion nozzle 3 within the mold 4 receives a braking force in a direction opposite to the flow direction due to the action of the magnetic field. Therefore, the flow of the molten steel M discharged from the discharge hole toward the widthwise ends of the solidified shell S is weakened. Further, the electromagnetic brake 4a can promote the floating of inclusions and the like in the molten steel M, and also plays the role of separating the inclusions and the like from the molten steel M.

The magnitude of the magnetic flux density of the magnetic field applied in the application process is preferably 1000 Gauss or more. If the magnetic flux density is 1000 Gauss or more, sufficient braking force acts on the molten steel M. In other words, when a magnetic field smaller than 1000 Gauss is applied to the molten steel M, the braking force acting on the molten steel M is insufficient. In this case, the temperature near the ends in the width direction of the slab 10 becomes high, and the completion of solidification is likely to be delayed, so that the internal quality of the slab 10 deteriorates.

The magnitude of the magnetic flux density of the magnetic field is not particularly limited as long as it is 1000 Gauss or more. However, the upper limit of the magnetic flux density is preferably 3000 Gauss. This is because when a magnetic field larger than 3000 Gauss is applied to the molten steel M, the temperature of the molten steel M near the immersion nozzle 3 increases. When continuously casting steel that is prone to surface cracking (for example, subperitectic steel), the temperature of the molten steel M near the immersion nozzle 3 increases. In this case, the initial solidification of the molten steel M within the mold 4 becomes non-uniform, and there is a possibility that vertical cracks may occur on the surface of the slab 10.

In the case of a general continuous casting method, the thickness of the solidified shell S of the slab 10 in the middle of solidification becomes non-uniform in the width direction. A region where the thickness of the solidified shell S is relatively small is a region of the slab 10 where completion of solidification is likely to be delayed. Hereinafter, a region where the thickness of the solidified shell S becomes relatively small when the slab 10 is cast using a general continuous casting method will be simply referred to as a delayed solidification region.

In the case of the continuous casting method of this embodiment, in the water amount adjustment step, the slab 10 is divided into a plurality of regions in the width direction, and the amount of secondary cooling water injected for each region is controlled. Specifically, the amount of water injected from the secondary cooling nozzles (in the present embodiment, the first nozzle 6A and the second nozzle 6B) is adjusted individually, and the amount of water is Inject a large amount of secondary cooling water. Hereinafter, among the amounts of water injected to each region across the width direction of the slab 10, the largest amount of water (for example, the amount of water injected to the delayed solidification area) and the smallest amount of water (for example, the amount of water injected to the delayed solidification area of the slab 10). The ratio of the amount of water injected to the area to be removed is simply called the water amount ratio. In the slab 10, the area corresponding to the solidification delayed portion is the above-mentioned non-contact portion 10b.

In the water amount adjustment step, the amount of secondary cooling water injected from the second nozzle 6B is set to be 1.5 times or more the amount of secondary cooling water injected from the first nozzle 6A. In other words, the amount of water injected to the non-contact portion 10b of the slab 10 is set to be 1.5 times or more the amount of water injected to the contact portion 10a of the slab 10. This can also be said to make the water ratio 1.5 or more. Generally, the non-contact portion 10b of the slab 10 is less likely to be cooled than the contact portion 10a, and the completion of solidification is likely to be delayed. In short, the non-contact portion 10b is a region corresponding to a delayed solidification portion. By supplying a large amount of secondary cooling water to the non-contact portion 10b, solidification in the non-contact portion 10b can be promoted.

However, if an excessive amount of secondary cooling water is supplied to a part of the slab 10, the surface temperature of that region becomes extremely low. Then, the surface temperature difference of the slab 10 in the width direction increases, and thermal stress is generated in the slab 10. In this case, depending on the steel type of the slab 10, surface cracks may occur in the slab 10. Therefore, the amount of water injected from the second nozzle 6B is preferably 4.0 times or less than the amount of water injected from the first nozzle 6A (water amount ratio is 4.0 or less).

Such water amount adjustment is performed for an appropriate range of the slab 10 in the casting direction. In a region where the thickness of the solidified shell S of the slab 10 is less than 30 mm, that is, directly under the mold 4, the slab 10 is at a very high temperature. If there is a difference in the surface temperature of the slab 10 in this region, there is a risk that surface cracks will occur in the slab 10 due to thermal stress. Further, when cooling the molten steel M in an unsolidified state with secondary cooling water, the solidified shell S acts as a thermal resistance, so it is difficult to promote solidification in a region where the solidified shell S is thick. Therefore, the water amount adjustment is preferably performed on the slab 10 in the range from when the thickness of the solidified shell S reaches 30 mm to 70 mm in the casting direction. A method for measuring the thickness of the solidified shell S will be described later.

In order to control the amount of secondary cooling water, the more finely the slab 10 is divided in the width direction, the more finely the amount of water supplied to the slab 10 can be controlled, but the control becomes more complicated and the cost required for control increases. becomes larger. In the slab 10, the dimension in the width direction of each divided area is preferably 150 to 250 mm. This is approximately the same size as the width direction dimension of the bearing portion 5b of the support roll 5.

In continuous casting of steel, when the arrangement of the roll body portion 5a and bearing portion 5b of the support roll 5 is symmetrical in the width direction of the slab 10, the solidification delayed portion is also symmetrical in the width direction. In this case, the amount of secondary cooling water supplied by the slab 10 may also be symmetrical in the width direction.

In the rolling process, the slab 10 is lightly rolled down in the thickness direction perpendicular to the width direction using a plurality of light rolling rolls 7 provided on the light rolling band 71 . Light reduction is performed after the center solid fraction of the slab 10 reaches 0.1 until the center solid fraction reaches 1.0. That is, at the inlet 71i of the light reduction zone 71, the center solid fraction of the slab 10 is 0.1, and at the outlet 71o of the light reduction zone 71, the center solid fraction of the slab 10 is 1.0. Even if the region of the slab 10 where the center solid fraction is smaller than 0.1 is lightly reduced, this region will not be affected in reducing the center segregation and porosity that are generally formed at the final stage of solidification, so this region is lightly reduced. There's no need. Furthermore, even if light reduction is applied to the area where the center solid fraction of the slab 10 is 1.0, that is, the area where the center of the thickness of the slab 10 is completely solidified, it has little effect on reducing center segregation and porosity. There is no need to lightly reduce this area.

In the rolling step, if the rolling speed in the light rolling zone 71 is lower than 0.5 mm/min, the rolling amount will be insufficient for the solidification shrinkage of the slab 10, which will worsen center segregation and porosity. Furthermore, if the rolling speed is higher than 1.3 mm/min, the rolling amount will be too large relative to the solidification shrinkage of the slab 10, and the molten steel M will flow backwards from downstream to upstream in the casting direction, and the flow of concentrated molten steel will cause the center to Segregation worsens. Therefore, the rolling speed of the slab 10 in the rolling step is preferably 0.5 mm/min or more and 1.3 mm/min or less.

In the continuous casting method according to the present embodiment, in the water amount adjustment step, the amount of water injected from the secondary cooling nozzles (first nozzle 6A and second nozzle 6B) is adjusted depending on the solidification state of the slab 10 in the width direction. do. The solidification state of the slab 10 in the width direction largely depends on the arrangement of the support rolls 5 of the continuous casting machine 1, and the solidification state will not change unless the casting conditions are significantly changed. Therefore, once the cross section of the slab 10 cast using the continuous casting machine 1 is examined, the solidification state in the width direction of the slab 10 cast using the same continuous casting machine 1 can be estimated. If the solidification state of the slab 10 can be estimated, the amount of water injected from the nozzle can be adjusted in advance so as to supply a large amount of secondary cooling water to the solidification delayed portion. An example of the estimation method will be described below.

Electromagnetic stirring is performed on the slab 10 within a range of 5 to 20 m from the meniscus (molten metal level in the mold 4), and a cross-sectional sample of the slab 10 is taken. The sample is corroded with hydrochloric acid to reveal a white band produced by stirring the unsolidified part. Thereafter, by measuring the distance from the surface of the slab 10 to the white band in the width direction of the slab 10, the solidification profile of the slab 10, that is, the thickness of the solidified shell S in the width direction of the slab 10, can be determined. You can get the transition.

[effect]
In the continuous casting method according to the present embodiment, in the application process, a magnetic field having a magnetic flux density of 1000 Gauss or more is applied to the molten steel M in the mold 4 by the electromagnetic brake 4a. Thereby, the flow of the molten steel M discharged from the discharge hole of the immersion nozzle 3 toward the widthwise end of the solidified shell S within the mold 4 can be weakened. Further, in the continuous casting method according to the present embodiment, in the water amount adjustment step, the amount of water injected to the non-contact portion 10b of the slab 10 is set to be 1.5 times or more the amount of water injected to the contact portion 10a of the slab 10. . Therefore, the non-contact portion 10b corresponding to the delayed solidification portion can be sufficiently cooled. In this way, by controlling both the flow of the molten steel M in the mold 4 and the amount of secondary cooling water, the thickness of the solidified shell S of the slab 10 can be made uniform in the width direction. By lightly rolling down this slab 10 in the thickness direction in the rolling process, center segregation and porosity can be reduced.

In the continuous casting method according to the present embodiment, not only the amount of secondary cooling water is controlled in the width direction, but also the solidified shell S of the slab 10 is controlled by controlling the flow of the molten steel M in the mold 4 using the electromagnetic brake 4a. Make the thickness uniform in the width direction. In this case, compared to the case where the thickness of the solidified shell S of the slab 10 is made uniform in the width direction only by controlling the amount of secondary cooling water, the surface temperature of the slab 10 is less likely to decrease. Thereby, generation of cracks on the surface of the slab 10 can be suppressed.

[Second embodiment]
FIG. 3 is a cross-sectional view of the continuous casting machine 1 of the second embodiment as viewed along the casting direction. FIG. 3 shows the lower part of the mold 4. Referring to FIG. 3, in the second embodiment, the continuous casting machine 1 includes a third nozzle 6C. The third nozzle 6C injects secondary cooling water to the ends of the slab 10 in the width direction.

In the continuous casting method according to the present embodiment, in the water amount adjustment step, the amount of water injected from the third nozzle 6C is set to be 1.5 times or more the amount of water injected from the first nozzle 6A. In short, the amount of water injected to the widthwise ends of the slab 10 is set to be at least 1.5 times the amount of water injected to the contact portion 10a of the slab 10. Although not particularly limited, the amount of water injected from the third nozzle 6C may be the same as the amount of water injected from the second nozzle 6B.

Generally, the ends in the width direction of the slab 10 tend to reach high temperatures because the high temperature molten steel M is supplied from the discharge hole in the mold 4, and the completion of solidification tends to be delayed. In short, in the slab 10, the widthwise end portion corresponds to a delayed solidification portion, similar to the non-contact portion 10b. According to the continuous casting method according to the second embodiment, the widthwise ends of the slab 10 can be sufficiently cooled. Thereby, center segregation and porosity can be further reduced.

In order to confirm the effects of the continuous casting method according to the embodiment, the following tests were conducted and the results were evaluated. Specifically, the Mn segregation degree, porosity volume ratio, and surface cracking of the slab obtained by continuous casting were evaluated.

The slab used in this test was manufactured using a continuous casting machine shown in FIG. The mold was a water-cooled copper mold. The length of the mold was 800 mm. The cross section of the mold was rectangular. The entrance to the light pressure zone was located 16 m from the meniscus (molten metal level in the mold). The slab was lightly rolled down in the thickness direction with a light rolling roll at a rolling speed of 0.8 mm/min from when the central solid fraction reached 0.1 until the central solid fraction reached 1.0. The center temperature and solid fraction of the slab were calculated by two-dimensional solidification analysis in the thickness direction and width direction of the slab. The specific amount of cooling water injected from the secondary cooling nozzle was set to 0.5 to 1.5 L/kg-steel.

The main chemical composition of the slab used in this test was C: 0.15%, Si: 0.19%, Mn: 0.90%, P: 0.011%, and S: 0.003%. there were.

Before conducting the test, the solidification profile of the slab cast by the continuous casting machine used in the test was measured. As described above, the solidification profile was determined by exposing a white band on a cross-sectional sample of the slab and measuring the distance from the surface of the slab to the white band in the width direction. As a result, the position in the width direction of the solidification delayed portion of the slab cast by the continuous casting machine was identified in advance.

The Mn segregation degree of the slab was investigated using the following procedure. The slab was cut at the center in the width direction, and a sample was taken so as to include an area of 40 mm in the casting direction and 40 mm in the thickness direction from the center of the cut surface in the thickness direction. A surface analysis of the Mn concentration was performed on this sample using an EPMA (Electron Probe Micro Analyzer). Mn concentrations were integrated over a width of 2 mm along the casting direction centering on the position with the highest Mn concentration, and the averaged value was taken as the maximum Mn concentration (Cmax). The value (Cmax/C0) obtained by dividing the maximum Mn concentration (Cmax) by the Mn concentration (C0) of the bulk composition of the slab was defined as the Mn segregation degree. The Mn concentration (C0) in the bulk composition was determined by taking a sample for analysis from the slab and conducting chemical analysis.

The porosity volume of the slab was investigated using the following procedure. A sample measuring 50 mm in the casting direction, 100 mm in the width direction, and 7 mm in the thickness direction was taken from the center of the slab in the thickness direction. Samples were taken from 16 locations along the width direction of the slab. For each sample, the density ρ was measured by the solid density and specific gravity measurement method specified in JIS Z 8807. Then, the porosity volume V (cm ³ /g) per unit weight was determined using the following formula (1). The density ρ ₀ in equation (1) was the density obtained by taking a sample from the 1/4 thickness part of the slab in the same manner as above and measuring it in the same manner as above. The value of the maximum porosity volume V among the porosity volumes V measured for each sample was defined as the porosity volume V of the slab.

The presence or absence of surface cracks in the slab was investigated using the following procedure. The surface of the slab was cut with a grinder, scale, etc. were removed, and a penetrant test was conducted to visually confirm the presence or absence of cracks on the surface of the slab. If there were unremovable cracks with a depth of 5 mm or more on the surface of the slab, the test piece was rejected; otherwise, it was passed.

Table 1 shows the test conditions and test results.

In Table 1, the test conditions include the thickness of the slab made by continuous casting, the casting speed, the water amount ratio, the magnetic flux density of the magnetic field of the electromagnetic brake, and the shell thickness deviation ratio. Here, the shell thickness deviation ratio is an index of solidification non-uniformity in the width direction of the slab, and was investigated using the following procedure. Electromagnetic stirring was performed on the slab, and the sample was corroded with hydrochloric acid to reveal a white band. The distance from the surface of the slab to the white band was measured at a pitch of 100 mm in the width direction of the slab. The difference between the largest solidified shell thickness and the smallest solidified shell thickness measured along the width direction was determined, and this was taken as the shell thickness deviation. The shell thickness deviation ratio is an index for comparing the shell thickness deviation between slabs of the same thickness. Select a standard slab from among multiple slabs, and use the shell thickness deviation of each slab as the standard. The value divided by the shell thickness deviation of the slab was taken as the shell thickness deviation ratio.

It is known that if the Mn segregation degree of the obtained slab is 1.4 or less, toughness can be ensured in the product (steel plate) after rolling the slab. If the degree was 1.4 or less, it was rated as "excellent," otherwise it was rated as "poor." In addition, it is known that if the porosity volume ratio is 0.8 or less, defects will be rendered harmless in the product (steel plate) after rolling the slab, so if the porosity volume ratio is 0.8 or less, "Excellent", otherwise "Unacceptable". Here, the porosity volume ratio is an index for comparing the porosity volume between slabs of the same thickness, and the porosity volume of each slab is selected as a reference slab from among multiple slabs, and the porosity volume of each slab is used as the standard. The value divided by the porosity volume of the slab was taken as the porosity volume ratio.

Furthermore, in Table 1, if there were unremovable cracks with a depth of 5 mm or more on the surface of the slab, it was rated "unsatisfactory", otherwise it was rated "excellent". Then, the Mn segregation degree, porosity volume ratio, and surface crack evaluation were comprehensively evaluated, and if all evaluations passed, the overall evaluation was "Excellent", otherwise, it was "Unsatisfactory".

For Invention Examples 1 to 4 and Comparative Examples 1 to 5, which correspond to slabs with a thickness of 250 mm, the shell thickness deviation ratio and porosity volume ratio of each slab were determined using the slab of Comparative Example 1 as a reference.

As shown in Table 1, Invention Examples 1 to 4 all satisfied the conditions defined in the above embodiment. Therefore, in Invention Examples 1 to 4, slabs with good Mn segregation degree and porosity volume ratio were obtained. Furthermore, no irremovable cracks were found on the surface of this slab. In other words, it was possible to suppress surface cracking of the slab while reducing center segregation and porosity.

In Comparative Example 1, the water amount ratio was smaller than the conditions specified in the above embodiment. In this case, the shell thickness deviation increased, and the Mn segregation degree and porosity volume ratio deteriorated. Comparative Example 2 had a small water ratio similar to Comparative Example 1 with respect to the conditions specified in the above embodiment, and furthermore, no magnetic field was applied to the molten steel in the mold. In this case, the shell thickness deviation became larger, and the Mn segregation degree and porosity volume ratio deteriorated.

In Comparative Example 3 and Comparative Example 4, no magnetic field was applied to the molten steel in the mold under the conditions specified in the above embodiment. However, in Comparative Example 3 and Comparative Example 4, different water ratios were used within the range of conditions specified in the above embodiment. In these cases, the shell thickness deviation became somewhat large, and the Mn segregation degree and porosity volume ratio deteriorated.

In Comparative Example 5, the water amount ratio was larger than the conditions specified in the above embodiment, and furthermore, no magnetic field was applied to the molten steel in the mold. In this case, because the water ratio was too high, cracks that could not be removed occurred on the surface of the slab.

Regarding Inventive Example 5 and Comparative Example 6, which correspond to slabs having a thickness of 300 mm, the shell thickness deviation ratio and porosity volume ratio of each slab were determined using the slab of Comparative Example 6 as a reference.

Invention Example 5 all satisfied the conditions defined in the above embodiment. Therefore, in Invention Example 5, a slab with good Mn segregation degree and porosity volume ratio was obtained. Furthermore, no irremovable cracks were found on the surface of this slab. In other words, it was possible to suppress surface cracking of the slab while reducing center segregation and porosity. On the other hand, in Comparative Example 6, the water amount ratio was smaller than the conditions specified in the above embodiment. In this case, the solidification delayed portion could not be sufficiently cooled, and the Mn segregation degree and porosity volume ratio deteriorated.

Regarding Inventive Example 6 and Comparative Example 7, which correspond to slabs having a thickness of 370 mm, the shell thickness deviation ratio and porosity volume ratio of each slab were determined using the slab of Comparative Example 7 as a reference.

Invention Example 6 all satisfied the conditions defined in the above embodiment. Therefore, in Invention Example 6, a slab with good Mn segregation degree and porosity volume ratio was obtained. Furthermore, no irremovable cracks were found on the surface of this slab. In other words, it was possible to suppress surface cracking of the slab while reducing center segregation and porosity. On the other hand, in Comparative Example 7, the water amount ratio was smaller than the conditions specified in the above embodiment. In this case, the solidification delayed portion could not be sufficiently cooled, and the Mn segregation degree and porosity volume ratio deteriorated.

The embodiments of the present disclosure have been described above. However, the embodiments described above are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the embodiments described above, and the embodiments described above can be modified and implemented as appropriate without departing from the spirit thereof.

1: Continuous casting machine 4: Mold 5: Support roll 5a: Roll body 5b: Bearing 6A: First nozzle 6B: Second nozzle 6C: Third nozzle 7: Light reduction roll 10: Slab S: Solidified shell

Claims (2)

a mold including an electromagnetic brake; a support roll installed on the downstream side of the mold in the casting direction and divided into a plurality of roll body parts in the width direction of the slab, the roll body parts being connected to each other by bearing parts; a first nozzle that injects secondary cooling water to a region of the slab that corresponds to the roll body; a second nozzle that injects secondary cooling water to a region of the slab that corresponds to the bearing section; A continuous casting method for steel using a continuous casting machine comprising: a light reduction roll installed downstream of the support roll in the casting direction,
an application step of applying a magnetic field with a magnetic flux density of 1000 Gauss or more to the molten steel in the mold using the electromagnetic brake;
A water amount adjustment step in which the amount of water injected from the second nozzle is 1.5 times or more the amount of water injected from the first nozzle until the thickness of the solidified shell of the slab reaches 30 mm and reaches 70 mm;
From the time the center solid fraction of the slab reaches 0.1 until the center solid fraction reaches 1.0, the slab is rolled at a speed of 0.5 mm/min or more and 1.3 mm/min with the light reduction roll. A rolling step of lightly rolling down the slab in the thickness direction perpendicular to the width direction at the following rolling speed;
A continuous casting method comprising: The continuous casting method according to claim 1,
The continuous casting machine further includes a third nozzle that injects secondary cooling water to an end in the width direction of the slab,
In the water amount adjustment step, the continuous casting method includes making the amount of water injected from the third nozzle 1.5 times or more the amount of water injected from the first nozzle.

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