EP4711060A1

EP4711060A1 - Slab manufacturing method

Info

Publication number: EP4711060A1
Application number: EP24825715.6A
Authority: EP
Inventors: Keigo TOISHI; Norichika ARAMAKI
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-06-20
Filing date: 2024-06-04
Publication date: 2026-03-18
Also published as: KR20260006676A; JP7597281B1; JPWO2024262308A1; CN121285438A

Abstract

Provided is a method for manufacturing a slab that enables the production of a slab of appropriate quality. The method for manufacturing a slab manufactures a slab of a predetermined size by using a continuous casting machine. The method for manufacturing a slab includes a soft reduction region setting step of setting a soft reduction region in the continuous casting machine, the soft reduction region containing a starting point for performing soft reduction on the slab and an endpoint at which the soft reduction is terminated, the soft reduction region being set based on a condition of cooling water for cooling the slab, in the continuous casting machine, a casting speed of slab, and a solid phase ratio of the slab determined from a composition of the slab; a reduction gradient setting step of setting a reduction gradient for the soft reduction region based on a thickness of the slab, a width of the slab, and the casting speed of slab; a secondary cooling intensity setting step of setting a first intensity section and a second intensity section, which are sections for performing secondary cooling in the soft reduction region, based on the solid phase ratio of the slab, the second intensity section being a section in which the secondary cooling is performed with a higher intensity than in the first intensity section; and a continuous casting step of performing continuous casting by using the set soft reduction region, reduction gradient, first intensity section, and second intensity section.

Description

Technical Field

The present invention relates to a method for manufacturing a slab having a thickness of 350 mm or greater and a ratio of a width to the thickness of 1.6 to 7.2, in a continuous casting machine.

Background Art

Continuous casting of steel involves cooling and solidifying molten steel to form a slab. The slab is withdrawn in a casting direction. In a final process of solidification of the molten steel, a so-called suction flow occurs, in which unsolidified molten steel, also referred to as an unsolidified layer, flows in a slab withdrawal direction along with solidification shrinkage. The unsolidified layer contains solute elements, such as carbon (C), phosphorus (P), sulfur (S), and manganese (Mn), that are concentrated therein. When such solute elements flow to a central portion of the slab, so-called center segregation occurs when the unsolidified layer solidifies.
Center segregation degrades the quality of steel products, in particular, steel plates. Steel plates, for example, are used as materials of line pipes for transporting oil or natural gas. When segregation exists in a material of a line pipe, the center segregation may act as initiation sites for hydrogen induced cracking, which may be caused by an action of sour gas.
In cases where a steel plate is used in offshore structures, storage tanks, oil tanks, and the like, a problem similar to the problem with the line pipe material application also occurs. In recent years, steel products have been used under low-temperature conditions or in severe use environments, such as highly corrosive environments. Regarding such severe use environments, there is a need to enhance the quality of steel plates, and, therefore, a need to reduce center segregation in slabs is ever increasing.
Steel materials are manufactured by a rolling step in which a slab manufactured by a continuous casting step is rolled. In the related art, attempts are being made to reduce center segregation in slabs during the process from the continuous casting step to the rolling step.
A known method for mitigating the center segregation of slabs is a technique of soft reduction at a final stage of solidification; the technique involves subjecting the slab, which includes an unsolidified layer, to reduction rolling in a continuous casting machine. The technique of soft reduction at the final stage of solidification is a technique for manufacturing a slab including gradually reduction-rolling the slab being processed by continuous casting, by using reduction rolls disposed near a final solidification point of the slab. The technique of soft reduction at the final stage of solidification is carried out at a reduction speed approximately corresponding to an amount of solidification shrinkage. Manufacturing a slab with the technique of soft reduction at the final stage of solidification can inhibit the formation of porosities in a central portion of the slab and also inhibit the flow of solute-enriched molten steel into the central portion. Consequently, center segregation in the slab can be inhibited.
To effectively prevent the occurrence of center segregation in a slab by using the technique of soft reduction at the final stage of solidification, it is important to appropriately set a starting time and an ending time of the period during which soft reduction is applied to the slab and also to appropriately set an amount of reduction for the soft reduction.
Patent Literature 1, for example, discloses a continuous casting method including applying soft reduction to a slab, and the method determines the amount of reduction per unit time for a zone in which the soft reduction is applied. In the case of Patent Literature 1, the amount of reduction is determined by using a surface temperature of the slab, which is a temperature at the start of the reduction rolling, and a thickness of the unsolidified layer of the slab, which is a thickness at a position of the reduction rolling.
Patent Literature 2 and Patent Literature 3 disclose a continuous casting method including continuously casting a bloom while reduction-rolling the bloom with multiple roll pairs, and the method includes increasing a reduction speed in accordance with a solid phase ratio in a thickness-wise central portion of the bloom. Note that in the cases of Patent Literature 2 and Patent Literature 3, reduction rolling is performed in a region corresponding to a time period starting from a time at which a temperature corresponding to a solid phase ratio in the thickness-wise central portion of the bloom of 0.1 to 0.3 is reached and ending at a time at which a temperature corresponding to a limit of fraction solid for liquid flow is reached.
Patent Literature 4 discloses a method for continuously casting a steel including performing continuous casting while applying reduction to a bloom, and the method includes adjusting rolling conditions based on information regarding a cross-sectional shape with the cross section being perpendicular to a longitudinal direction of the bloom and information regarding a shape of an unsolidified region in the cross section.

Citation List

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 8-132203
PTL 2: Japanese Unexamined Patent Application Publication No. 3-90263
PTL 3: Japanese Unexamined Patent Application Publication No. 3-90259
PTL 4: Japanese Unexamined Patent Application Publication No. 2003-71552

Summary of Invention

Technical Problem

Unfortunately, a problem exists in that even when any of the disclosures of Patent Literature 1 to 4 is used, slabs of appropriate quality may not be obtained, depending on a size of the slabs, and, therefore, there is a need for further improvement in the method for manufacturing a slab that uses a continuous casting machine.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for manufacturing a slab that uses a thickness continuous casting machine and enables the production of a slab of appropriate quality regardless of a size of the slab.

Solution to Problem

The present invention has the following features in order to solve the problem described above.

[1] A method for manufacturing a slab having a thickness of 350 mm or greater and a ratio of a width to the thickness of 1.6 to 7.2, the method using a continuous casting machine, the method including:
- a soft reduction region setting step of setting a soft reduction region in the continuous casting machine, the soft reduction region containing a starting point for performing soft reduction on the slab and an endpoint at which the soft reduction is terminated, the soft reduction region being set based on a condition of cooling water for cooling the slab, in the continuous casting machine, a casting speed of slab, and a solid phase ratio of the slab determined from a composition of the slab;
- a reduction gradient setting step of setting a reduction gradient for the soft reduction region based on a thickness of the slab, a width of the slab, and the casting speed of slab;
- a secondary cooling intensity setting step of setting a first intensity section and a second intensity section, which are sections for performing secondary cooling in the soft reduction region, based on the solid phase ratio of the slab, the second intensity section being a section in which the secondary cooling is performed with a higher intensity than in the first intensity section; and
- a continuous casting step of performing continuous casting by using the set soft reduction region, reduction gradient, first intensity section, and second intensity section.
[2] The method for manufacturing a slab according to [1], wherein
- in the soft reduction region setting step, the soft reduction region is set by selecting, as the starting point, a position at which the solid phase ratio of a central portion in a thickness direction of the slab reaches 0.1, and, as the endpoint, a position at which the solid phase ratio of the central portion in the thickness direction of the slab reaches a limit of fraction solid for liquid flow,
- in the reduction gradient setting step, the reduction gradient for the soft reduction region is set to satisfy relationships of inequality (1) and equation (2), shown below, and
- in the secondary cooling intensity setting step, a water flow density for the secondary cooling in the second intensity section is set to satisfy inequality (3): $0.05 / (V \times α) < Z < 2.0 / (V \times α)$ $α = (β \times (D / Do) + γ)$ $W 1 \geq 10$
- where V is the casting speed of slab (m/min), α is a thickness coefficient (-), Z is the reduction gradient (mm/m), D is a thickness (mm) of the slab when the slab is directly under a casting mold, Do is a reference thickness (mm) of the slab when the slab is directly under the casting mold, W1 is the water flow density (L/m²/min), β and γ are coefficients determined by a width W2 (mm) of the slab 11, and β and γ are expressed as follows, depending on ranges of the width W2 of the slab:
  - when W2 ≤ 1300, β = -0.51, and γ = 1.54,
  - when 1300 < W2 ≤ 1700, β = -0.49, and γ = 1.56,
  - when 1700 < W2 ≤ 2100, β = -0.47, and γ = 1.58, and
  - when 2100 < W2, β = -0.45, and γ = 1.60.
[3] The method for manufacturing a slab according to [1] or [2], further including a total reduction amount setting step of setting a total reduction amount for the slab, based on the thickness of the slab, wherein
- in the total reduction amount setting step, the total reduction amount is set to satisfy a relationship of inequality (4), shown below: $Rt < (D / Do) \times (10 / α)$
- where Rt is the total reduction amount (mm) for the slab, D is a thickness (mm) of the slab when the slab is directly under a casting mold, Do is a reference thickness (mm) of the slab when the slab is directly under the casting mold, and α is a thickness coefficient (-).

Advantageous Effects of Invention

According to the present invention, a method for manufacturing a slab that uses a continuous casting machine enables the production of a slab of appropriate quality regardless of a size of the slab.

Brief Description of Drawings

[Fig. 1] Fig. 1 is a side view illustrating an overall configuration of a continuous casting machine.
[Fig. 2] Fig. 2 is a side view of roll segments.
[Fig. 3] Fig. 3 is a front view of a roll segment.
[Fig. 4] Fig. 4 is a flowchart illustrating process steps of a method for manufacturing a slab.

Description of Embodiments

The present inventors empirically found that regarding continuous casting for casting a slab by applying soft reduction to the slab, an optimal reduction speed for the region in which the reduction force is applied to the slab (hereinafter also referred to as "soft reduction zone") varies depending on a thickness of the slab.
The thickness of the slab is set based on the thickness of a post-rolling steel product and a reduction ratio for the rolling required by the specifications of the steel product. Accordingly, the thickness of a slab is set, for example, in accordance with the specifications of a steel product. When no slabs with the set thickness have been manufactured by casting that uses the soft reduction technique, it is necessary to newly set the reduction speed for soft reduction suitable for the thickness of the slab.
The reduction speed for soft reduction can be determined, for example, by a casting experiment performed by setting several levels of reduction gradients and using an actual machine. Setting the reduction speed requires a long time and a high cost. There is a need for the setting of the reduction gradient for soft reduction to be accomplished conveniently.
As referred to herein, the "reduction gradient" is a rate of change in a distance between opposing rolls (hereinafter also referred to as a "roll gap"), where the roll gap is set such that the distance gradually decreases toward a downstream side in a casting direction. The reduction gradient is typically expressed as a rate of change in the roll gap per m (mm/m). The reduction speed (mm/min) is the result of multiplying the reduction gradient (mm/m) by a casting speed of slab (m/min).
In the case of Patent Literature 1, attention is focused on a thickness of an unsolidified layer of the slab, and the thickness is used as an index for effectively performing soft reduction, to carry out continuous casting. In the case of Patent Literature 1, the focus of the attention was made based on the knowledge that a ratio of an amount of reduction set for reduction rolls that is transmitted to a solid-liquid interface of the slab decreases when reduction rolling occurs more downstream with respect to the casting direction, that is, in a state in which the thickness of the unsolidified layer in the slab is smaller (the ratio is hereinafter also referred to as "reduction efficiency").
According to experiences of the present inventors, however, center segregation becomes manifest in a region of a central portion of the slab in which the thickness of the unsolidified layer is approximately 10 mm or less. According to the disclosure of Patent Literature 1, in a comparison between thicknesses of the unsolidified layer of 10 mm and 0 mm, a difference in the required reduction speed is approximately 10%. With such a difference in the reduction speed, sufficient inhibition of center segregation may not be achieved. Furthermore, the "Examples" section of Patent Literature 1 describes a test result of one size of slab having a thickness (250 mm), and, therefore, it is unknown whether the optimal reduction conditions described in Patent Literature 1 are effective for different thicknesses of slabs.
In the cases of Patent Literature 2 and 3, a test was conducted on three sizes of blooms measuring 300 mm × 500 mm, 162 mm × 162 mm, and 380 mm × 560 mm (thickness × width). Patent Literature 2 and 3 both relate to softreduction casting of a bloom. Blooms have a lower ratio of width to thickness (width/thickness) in a cross section perpendicular to the withdrawal direction than slabs. Accordingly, in the case of blooms, the reduction efficiency of the soft reduction at the final stage of solidification is lower than that of slabs. In the process of casting a bloom, the amount of reduction is set such that the amount increases as the process approaches the final stage of solidification. In the examples of the blooms of Patent Literature 2 and 3, the amount of reduction is approximately two to three times larger than that of the example of the slab of Patent Literature 1. A problem exists in that the reduction conditions of Patent Literature 2 and 3 cannot be used for the soft reduction of slabs.
In addition, in the cases of Patent Literature 1 to 3, the reduction gradient for the soft reduction zone is varied along the casting withdrawal direction. Accordingly, in these examples, a problem exists in that the setting of the roll gap for slab or bloom support rolls is complex, and that, therefore, the process in an actual machine requires a complex structure of the equipment.
Patent Literature 4 is directed to a bloom and discloses that reduction conditions are set based on information regarding a shape of a cross section perpendicular to a longitudinal direction of the bloom, that is, based on a width and a thickness of the bloom.
In the case of Patent Literature 4, the reduction conditions are set by using the ratio between the width and the thickness of the bloom and the ratio between the width and the thickness of the unsolidified portion of the bloom; that is, the thickness itself of the bloom is not used for the setting of the reduction conditions.
Regarding blooms, there are cases where a shape of the unsolidified layer of the blooms may become flat in a vertical direction, depending on a cooling ratio between an upper surface and a lower surface of the blooms in a continuous casting machine. Furthermore, there are cases where the shape of the unsolidified layer of the blooms may become flat in a left-and-right direction, depending on the cooling ratio between a left surface and a right surface of the blooms. Regarding these cases of flatness of the unsolidified layer of the blooms, there is a concern that both of the cases, not just one of them, may occur. According to Patent Literature 4, a purpose is to enable optimal soft reduction even in cases where both of the cases of flatness occur.
In the case of slabs, in which a long side is significantly longer than a short side, the flatness of the unsolidified layer is likely to occur in the left-and-right direction of the slabs. Accordingly, a problem exists in that with Patent Literature 4, which is based on the assumption that the flatness of the unsolidified layer occurs in the left-and-right direction and vertical direction, it is impossible to effectively inhibit center segregation in slabs.
The present invention will be described in detail below with reference to the accompanying drawings. Fig. 1 is a side view of a continuous casting machine. As illustrated in Fig. 1, a continuous casting machine 100 includes a tundish 20, which is in the form of a container for storing molten steel 10, which is supplied from a ladle (not illustrated). A submerged entry nozzle 21, through which the stored molten steel 10 is discharged, is provided in a bottom portion of the tundish 20. In addition, a sliding nozzle 22, which is used to adjust a flow rate of the molten steel 10, is provided in the tundish 20.
The continuous casting machine 100 includes a casting mold 30, which is used to cool the molten steel 10 discharged from the tundish 20. The casting mold 30 is formed to have a tubular shape having openings at a top and a bottom and having a cavity for holding the molten steel 10. The cavity is formed of a pair of plate-shaped long sides, which are disposed to oppose each other, and a pair of plate-shaped short sides, which are disposed to oppose each other in a direction perpendicular to the surfaces that oppose each other of the long sides.
The molten steel 10 flowing from the opening at the top is cooled by the cavity of the casting mold 30 to form an outer shell of a slab 11. The slab 11 is withdrawn from the opening formed at the bottom of the casting mold 30. The cooling of the molten steel 10 that takes place in the casting mold 30 is also referred to as "primary cooling".
The slab 11 is a slab. The slab 11 includes an unsolidified layer 11a and a solidified shell 11b, which covers a periphery of the unsolidified layer 11a. The solidified shell 11b is the outer shell of the slab 11 formed by the cooling of the molten steel 10 in the casting mold 30.
The slab 11 has a thickness of 350 mm or greater. The thickness of the slab 11 is, for example, 350 to 600 mm. It is preferable that the slab 11 have a large thickness because the larger the thickness, the higher the reduction ratio that can be achieved. The thickness of the slab 11 can be set by the length of the short side at the opening formed at the bottom of the casting mold 30.
In the case of the related art, when the thickness of a slab 11 is 350 mm or greater, the frequency of occurrence of center segregation tends to increase. With a method of the present invention for manufacturing a slab, it is possible to inhibit the occurrence of center segregation in a slab 11 having a thickness of 350 mm or greater and a ratio of a width of the thickness of 1.6 to 7.2.
When the thickness of the slab 11 is 600 mm or less, it is possible to refine the microstructures of a central portion. In addition, in the continuous casting machine 100, which is of a vertical bending type, appropriate bending and straightening of the slab 11 can be achieved.
The slab 11 has a ratio of the width to the thickness (width/thickness) of, for example, 1.6 to 7.2. The width of the slab 11 can be set by the length of the long side at the opening formed at the bottom of the casting mold 30. Because of the ratio of the width to the thickness of 1.6 to 7.2, insufficient reduction and excessive reduction can be inhibited.
The continuous casting machine 100 includes slab support rolls 40, which are provided below the casting mold 30. The slab support rolls 40 include support rolls 41, guide rolls 42, and pinch rolls 43.
The support rolls 41 are formed to have a cylindrical shape and are disposed as pairs, each configured to hold a top surface and a bottom surface of the slab 11. The plurality of support rolls 41 is disposed in lines toward a downstream side in a casting direction.
The guide rolls 42 are formed to have a cylindrical shape and are disposed following the support rolls 41. The guide rolls 42 are disposed as pairs, each configured to hold the top surface and the bottom surface of the slab 11. The plurality of guide rolls 42 is disposed in lines toward the downstream side in the casting direction.
The pinch rolls 43 are formed to have a cylindrical shape and are disposed following the guide rolls 42. The pinch rolls 43 are disposed as pairs, each configured to hold the top surface and the bottom surface of the slab 11. The plurality of pinch rolls 43 is disposed in lines toward the downstream side in the casting direction.
The slab 11, which includes the solidified shell 11b formed in the casting mold 30, is subjected to secondary cooling that uses cooling water. Specifically, spray nozzles (not illustrated) are disposed between adjacent slab support rolls 40 (support rolls 41, guide rolls 42, and pinch rolls 43). The section in which the spray nozzles for spraying secondary cooling water are disposed may be referred to as a secondary cooling zone. Examples of spray nozzles that can be used include water spray nozzles and air mist spray nozzles.
The slab 11 is cooled by cooling water sprayed from the spray nozzles in the secondary cooling zone as the slab 11 moves in the casting direction. The cooling water used for the secondary cooling is also referred to as secondary cooling water.
The section where the slab support rolls 40 are provided includes a soft reduction region (hereinafter also referred to as a "soft reduction zone") 44, in which a pressure is applied in a thickness direction of the slab 11. The soft reduction zone 44 is one in which the support rolls 41, the guide rolls 42, and the pinch rolls 43, which are disposed to form opposing pairs, are disposed such that a distance between opposing rolls gradually decreases. In the soft reduction zone 44, the amount of reduction applied to the slab 11 approximately corresponds to, for example, the amount of solidification shrinkage.
The distance between paired rolls of the support rolls 41, the guide rolls 42, and the pinch rolls 43 is also referred to as a "roll gap". Regarding the soft reduction zone 44, a reduction gradient is set by adjusting the roll gap.
For example, the reduction gradient is adjusted by using an amount of narrowing of the roll gap per m (mm/m) in the casting direction. A speed of the reduction (mm/min) in the soft reduction zone 44 can be determined by multiplying the reduction gradient (mm/m) by a slab 11 withdrawal speed (m/min).
In the continuous casting machine 100 illustrated in Fig. 1, the soft reduction zone 44 is composed of two roll segments 45, which are sequentially disposed in the casting direction. Each of the roll segments 45 is composed of three pairs of slab support rolls 40.
The number of the roll segments 45 that constitute the soft reduction zone 44 is not particularly limited, and one or more roll segments 45 may be provided. In addition, the number of the slab support rolls 40 that constitute each of the roll segments 45 is not particularly limited, and two or more pairs of slab support rolls 40 may constitute each of the roll segments 45.
The continuous casting machine 100 includes transfer rolls 50, which are disposed following the slab support rolls 40. In addition, the continuous casting machine 100 includes a slab cutting machine 60, which is disposed above the transfer rolls 50. The slab cutting machine 60 operates at predetermined times to cut the slab 11 into predetermined lengths.
In the slab 11, solidification progresses into an inner portion and is completed at a final solidification point 13 of the soft reduction zone 44. After solidification is complete, the slab 11 is cut by the slab cutting machine 60.
Fig. 2 is a side view of the roll segments 45 of the continuous casting machine. Fig. 3 is a front view of the roll segments 45 of the continuous casting machine. The example illustrated in Fig. 2 and Fig. 3 is one in which three pairs of slab support rolls 40 constitute one roll segment 45.
The roll segments 45 include a pair of frames 46 and 47, which have a rectangular plate shape and are disposed to oppose each other in a vertical direction. A plurality of roll chocks 48 is disposed on opposing surfaces of the pair of frames 46 and 47. The slab support rolls 40 are supported on the roll chocks 48. The pair of frames 46 and 47 are supported on tie rods 49, which are disposed on four corners of the frames. One pair of the tie rods 49 is disposed at both ends on an upstream side in the casting direction. The other pair of the tie rods 49 is disposed at both ends on a downstream side in the casting direction.
Each of the tie rods 49 is provided with a worm jack WJ. The worm jack WJ is connected to a motor MT. The distance between the frames 46 and 47 can be adjusted by actuating the worm jack WJ by using the motor MT.
The adjustment of the reduction gradient can be carried out by adjusting the distance between opposing rolls of the slab support rolls 40. The distance between opposing rolls of the slab support rolls 40 is a value corresponding to the distance between the frame 46 and the frame 47. Accordingly, the reduction gradient can be adjusted by adjusting the distance between the frame 46 and the frame 47. An amount of movement of the frame 46 is an amount corresponding to an amount of actuation of the worm jack WJ. Accordingly, by preliminarily recording the amount corresponding to the amount of actuation of the worm jack WJ, it is possible to set the reduction gradient of the roll segment 45 in accordance with the amount of actuation of the worm jack WJ.
Desirably, the adjustment of the reduction gradient is carried out under conditions under which a load from the slab 11 is not applied to the slab support rolls 40. Such conditions include, for example, those in which the slab 11 is not present in the roll segment 45.
A method for manufacturing the slab 11 with the above-described continuous casting machine 100 will be described. Fig. 4 is a flowchart illustrating the method for manufacturing the slab 11 that uses the continuous casting machine 100.
As illustrated in Fig. 4, a soft reduction region setting step (step S01) is performed to set the soft reduction zone 44 in the continuous casting machine 100; the soft reduction zone 44 contains a starting point for performing soft reduction on the slab 11 and an endpoint at which the soft reduction is terminated.
In the soft reduction region setting step of step S01, the starting point for performing the soft reduction and the endpoint at which the soft reduction is terminated are set based on a condition of cooling water for cooling the slab 11, a slab 11 withdrawal speed, and a solid phase ratio of the slab 11 determined from a composition of the slab 11.
Specifically, it is desirable that the starting point of the soft reduction zone 44 that is set be a position corresponding to a solid phase ratio of a central portion in the thickness direction of the slab 11 of 0.1 or is a position that precedes the mentioned position. The central portion in the thickness direction of the slab 11 may be, for example, a region extending 15 mm from a center in the thickness direction of the slab 11. The endpoint at which the soft reduction is terminated that is set is desirably a position at which the solid phase ratio of the central portion in the thickness direction of the slab 11 reaches a limit of fraction solid for liquid flow.
The solid phase ratio is defined as a ratio that is 0 before solidification is started and is 1.0 at the time solidification is complete. Accordingly, in the continuous casting machine 100, the position at which the solid phase ratio of the central portion in the thickness direction of the slab 11 reaches 1.0 corresponds to a position at which solidification is complete.
The solid phase ratio can be determined from the condition of the cooling water for cooling the slab 11, the slab 11 withdrawal speed, and a composition of the slab 11. In addition, the solid phase ratio of the central portion can be estimated by determining a temperature distribution of the slab 11 by two-dimensional solidification heat transfer calculation and using the results. The solid phase ratio of the central portion in the thickness direction of the slab 11 may be a value estimated from the temperature of a surface of the slab 11.
The solidification heat transfer calculation may be carried out, for example, by commonly performed two-dimensional heat transfer analysis of solidification that takes into account in-mold cooling, the arrangement of spray nozzles and rolls in a secondary cooling zone and cooling by using them, and radiation-convection heat transfer cooling. Various methods for the two-dimensional heat transfer analysis of solidification have been proposed, and among them is a method described in Japanese Unexamined Patent Application Publication No. 2002-178117 . This method does not require a high calculation load and can be used for online real-time calculation.
When the soft reduction is begun after the solid phase ratio of the central portion in the thickness direction of the slab 11 exceeds 0.1, there is a possibility that the flow of solute-enriched molten steel may occur before the soft reduction. When the flow of solute-enriched molten steel occurs, center segregation may occur, that is, the effect of reducing center segregation cannot be sufficiently produced. Desirably, the soft reduction is begun before the solid phase ratio of the central portion in the thickness direction of the slab 11 reaches 0.1.
In general, the limit of fraction solid for liquid flow is considered to be 0.7 to 0.8. Desirably, the reduction rolling in the soft reduction zone 44 is continued until the solid phase ratio of the central portion in the thickness direction of the slab 11 reaches 0.7 to 0.8.
After the solid phase ratio of the central portion in the thickness direction of the slab 11 exceeds the limit of fraction solid for liquid flow, the unsolidified layer 11a does not move, and, therefore, the effect of the soft reduction diminishes. Accordingly, it is preferable that the soft reduction not be continued after the limit of fraction solid for liquid flow is reached.
When reduction rolling is performed in a state in which the solid phase ratio is greater than or equal to the limit of fraction solid for liquid flow, a strength of the slab 11 becomes higher than the strength exhibited when the solid phase ratio is less than the limit of fraction solid for liquid flow. Because of this, a high reaction force may be generated during soft reduction, and, therefore, the slab 11 may not be appropriately reduction-rolled.
Subsequently, a reduction gradient setting step (step S02) is performed to set the reduction gradient for the soft reduction zone 44 based on the thickness of the slab 11, the width of the slab 11, and the slab 11 withdrawal speed.
As is generally known, in cases where soft reduction is performed at a final stage of solidification of the slab 11, center segregation in the slab 11 is reduced. When soft reduction is performed, the solidified shell 11b of the slab 11 deforms under the action of the reduction rolling. When the solidified shell 11b deforms, the reduction rolling may not be accomplished at the set reduction speed. Consequently, the reduction efficiency may decrease, where the reduction efficiency is represented by the ratio of the amount of reduction that is transmitted to the solidification interface to the amount of reduction applied to the slab 11.
Regarding the reduction efficiency, there is a tendency for the reduction efficiency to decrease as the thickness of the solidified shell 11b increases. The larger the thickness of the slab 11, the larger the thickness of the solidified shell 11b, which results in a decrease in the reduction efficiency.
In a case where, for example, a slab 11 having a width of 2100 mm and a thickness of 350 to 600 mm, that is, having a ratio of the width to the thickness (width/thickness) of 3.5 to 6.0, is manufactured, conditions for the soft reduction can be determined in the following manner. The conditions for the soft reduction that are to be set may be the thickness of the slab 11, the reduction gradient for the soft reduction zone 44, the slab 11 withdrawal speed, and conditions for the secondary cooling.
The thickness of the slab 11, the reduction gradient for the soft reduction zone 44, and the slab 11 withdrawal speed are set to satisfy the relationships of inequality (1) and equation (2), shown below. The conditions for the secondary cooling are set to satisfy inequality (3), shown below. $0.05 / (V \times α) < Z < 2.0 / (V \times α)$ $α = (β \times (D / Do) + γ)$ $W 1 \geq 10$
In inequalities and equation (1) to (3), V is the casting speed of slab (m/min), α is a thickness coefficient (-), Z is the reduction gradient (mm/m), D is a thickness (mm) of the slab when the slab is directly under the casting mold, Do is a reference thickness (mm) of the slab when the slab is directly under the casting mold, W1 is the water flow density (L/m²/min), and β and γ are coefficients determined by a width W2 (mm) of the slab.
β and γ are expressed as follows, depending on ranges of the width W2 of the slab 11.
When W2 ≤ 1300, β = -0.51, and γ = 1.54.
When 1300 < W2 ≤ 1700, β = -0.49, and γ = 1.56.
When 1700 < W2 ≤ 2100, β = -0.47, and γ = 1.58.
When 2100 < W2, β = -0.45, and γ = 1.60.
Inequalities and equation (1) to (4) can be determined as follows. First, for the determination of inequality (1), an optimal range of the reduction gradient for the soft reduction zone 44, in the case of continuously casting a slab 11 having a thickness of 350 mm, may be determined. The optimal range of the reduction gradient can be determined, for example, by a casting experiment performed with an actual machine.
Next, a correction value associated with an influence of the thickness of the slab 11 may be determined. The correction value can be determined, for example, by a numerical simulation regarding deformation of the slab 11 that occurs during soft reduction. The numerical simulation can be carried out in a range of the thickness of the slab 11 of 150 to 600 mm. From the results of a simulation, a relationship between the thickness of the slab 11 and the reduction efficiency was determined, and the thickness coefficient α, which is the correction value, was calculated.
For a first-order approximation equation of the thickness of the slab 11, the coefficient of equation (2) for calculating the thickness coefficient α was obtained. The value of the thickness coefficient α decreases as the thickness D of the slab 11 increases. This indicates that the larger the thickness D of the slab, the lower the reduction efficiency.
The reference thickness Do of the slab 11 when the slab is directly under the casting mold is the thickness of the slab 11 corresponding to a thickness coefficient α of 1 as determined by equation (2), and in the case of a slab 11 having a width of 2100 mm, the reference thickness Do is 187 mm.
When the thickness of the slab 11 that is the casting target is different from the reference thickness of 187 mm, the reduction efficiency varies from the reduction efficiency associated with the reference thickness. The change in the reduction efficiency resulting from a change in the thickness of the slab 11 can be adjusted by adjusting the reduction gradient.
Specifically, when the reduction efficiency is lower than the reduction efficiency associated with the reference thickness, the reduction gradient is desirably adjusted to be larger, and when the reduction efficiency is higher than the reduction efficiency associated with the reference thickness, the reduction gradient is desirably adjusted to be smaller. Accordingly, equation (1), which is a relational equation regarding the casting speed of slab, the thickness coefficient α, and the reduction gradient, was obtained. $0.05 / (V \times α) < Z < 2.0 / (V \times α)$
By performing soft reduction with the reduction gradient that is in accordance with equation (1), which was obtained as described above, it is possible to achieve an appropriate reduction efficiency for the slab 11. Consequently, the formation of center segregation and porosities in the slab 11 can be prevented, and, further, the occurrence of inverted V segregation and internal cracking due to excessive reduction in the slab 11 can be prevented.
The width of the slab 11 that is cast by the continuous casting machine 100 varies in a wide range of 1000 to 2500 mm. Accordingly, for slabs 11 having a thickness of 350 to 600 mm, a width of 1000 to 2500 mm, and a ratio of the width to the thickness (width/thickness) of 1.6 to 7.2, the thickness coefficient α of the slabs in the entire ranges was determined.
During soft reduction in the soft reduction zone 44, resistance to reduction occurs mainly due to portions at the short sides of the slab 11 in which solidification is complete. The absolute values of the lengths of this portion in a width direction of the slab 11 are approximately equal regardless of the size of the width of the slab 11 provided that the thickness of the slab 11 does not vary.
In regions including an unsolidified layer, the resistance to reduction is low because of the presence of the unsolidified layer. This resistance to reduction is negligibly low compared with the portions at both ends at the short sides of the slab 11 in which solidification is complete.
For example, in the case of a slab 11 with a width of 1600 mm, the portions at the short sides in which solidification is complete have a larger proportion to the width than those of a slab 11 with a width of 2100 mm. Accordingly, the resistance to reduction of the slab 11 with a width of 1600 mm is higher than that of the slab 11 with a width of 2100 mm.
When soft reduction is performed on a slab 11 with a width of 1600 mm, with the same reduction gradient as that of a slab 11 with a width of 2100 mm, a reaction force due to resistance to reduction may exceed a preset stress of a disc spring. In this case, the roll gap may be expanded during the soft reduction, and, consequently, the actual reduction gradient may become smaller than the set reduction gradient.
For slabs 11 with a width of 1000 mm to 2500 mm, a numerical simulation similar to the above-described numerical simulation was performed to determine the thickness coefficient α of each of them.
Thus, for a first-order approximation equation of the thickness of the slabs 11, equation (2) regarding the thickness coefficient α was obtained. β and γ are coefficients determined by the width W2 (mm) of the slab 11. $α = (β \times (D / Do) + γ)$
β and γ are as follows, depending on the width W2 (mm) of the slab 11.
When 1000 ≤ W2 ≤ 1300, β = -0.51, and γ = 1.54.
When 1300 < W2 ≤ 1700, β = -0.49, and γ = 1.56.
When 1700 < W2 ≤ 2100, β = -0.47, and γ = 1.58.
When 2100 < W2 ≤ 2500, β = -0.45, and γ = 1.60.
Furthermore, under various casting conditions for the continuous casting operation, the thickness of the solidified shell 11b and the solid phase ratio of the central portion in the thickness direction of the slab 11 are preliminarily determined, for example, by two-dimensional heat transfer analysis of solidification.
The amount of secondary cooling water or the slab 11 withdrawal speed is adjusted such that the solid phase ratio of the central portion in the thickness direction of the slab 11 at the time the slab 11 enters the soft reduction zone 44 becomes 0.1 or less, and that the solid phase ratio of the central portion in the thickness direction of the slab 11 at the time the slab 11 exits the soft reduction zone 44 reaches or exceeds the limit of fraction solid for liquid flow.
Subsequently, a total reduction amount setting step (step S03) is performed to set a total reduction amount for the slab 11. In the total reduction amount setting step of step S03, the total reduction amount (Rt) for the slab 11 and the thickness of the slab 11 are preferably set to be within a range that satisfies the relationship of inequality (4), shown below. $Rt < (D / Do) \times (10 / α)$
Rt is the total reduction amount (mm).
While soft reduction has the effect of preventing the flow of solute-enriched molten steel at the final solidification portion, the reduction rolling causes the slab 11 to deform. Thus, soft reduction may result in the occurrence of internal cracking at the solidification interface. It is known that internal cracking occurs when a cumulative value of strain applied to the solidification interface reaches a value greater than or equal to a certain value. Accordingly, it is preferable that the total reduction amount for the slab 11 be set. Thus, setting the total reduction amount enables the inhibition of internal cracking in the slab 11. The total reduction amount setting step of step S03 is a step that can be optionally performed.
Furthermore, a secondary cooling intensity setting step (step S04) is performed to set a first intensity section and a second intensity section, which are sections for performing secondary cooling in the soft reduction zone 44 region, based on the solid phase ratio of the slab 11, the second intensity section being a section in which the secondary cooling is performed with a higher intensity than in the first intensity section.
When the thickness of the slab 11 is 350 mm or greater, a temperature gradient of the central portion of the slab 11 may increase, which may result in coarse solidified microstructures, which, in turn, may exacerbate center segregation. As a result, the effect of soft reduction alone is not sufficient for improving an internal quality.
Accordingly, the first intensity section and the second intensity section are set in the soft reduction zone 44, and, consequently, fine solidified microstructures can be obtained in the slab 11. The first intensity section to be set may be, for example, a section starting from the starting point of the soft reduction zone 44 and ending at a point corresponding to a solid phase ratio of the central portion in the thickness direction of the slab 11 of 0.2.
The second intensity section to be set may be, for example, a section starting from a point corresponding to a solid phase ratio of the central portion in the thickness direction of the slab 11 of 0.2 and ending at 0.8. In the second intensity section, it is preferable that the cooling be performed, for example, with a water flow density of 10 (L/m²/min) or greater. Cooling the slab 11 with such a water flow density makes it possible to obtain fine solidified microstructures.
Lastly, a continuous casting step (step S05) is performed to perform continuous casting by using the soft reduction region, the reduction gradient, the first intensity section, and the second intensity section that have been set in the steps of step S01 to 04, described above.
The method of the present invention for manufacturing a slab, described above, enables the production of a slab of appropriate quality regardless of a size of the slab. In particular, the soft reduction zone 44 and the reduction gradient can be set by the combination of inequalities and equation (1) to (3), shown above, and thus, optimal reduction conditions can be determined conveniently. Accordingly, it is possible to inhibit the occurrence of center segregation due to an insufficient amount of reduction and the occurrence of internal cracking due to an excessive amount of reduction. Consequently, it is possible to quickly address a need for manufacturing steel products of various specifications, without spending much time or incurring high cost, which may be the case when an experiment with several levels is performed with an actual machine.

EXAMPLES

Slabs of Invention Examples 1 to 9 and Comparative Examples 1 to 9 were manufactured by casting in a continuous casting machine, and an investigation was made as to a degree of center segregation, the presence or absence of porosities, and the presence or absence of internal cracks, in the slabs. The material of the slabs of Invention Examples 1 to 9 and Comparative Examples 1 to 9 was a lowcarbon aluminum killed steel. The prepared slabs of Invention Examples 1 to 9 and Comparative Examples 1 to 9 had a thickness of 350 mm, 400 mm, or 450 mm and a width of 2000 mm.

Casting conditions for the slabs of Invention Examples 1 to 9 and Comparative Examples 1 to 9 are shown in Table 1. The continuous casting machine used in the test was similar to the continuous casting machine 100, illustrated in Fig. 1.

[Table 1]

	Thickness of Slab (mm)	Ratio of Width to Thickness (Width/Thickness)	Specific Water Flow (L/kg)	Withdrawal Speed (m/min) V	Reduction Gradient (mm/m) Z	Set Value of Total Reduction Amount (mm) Rt	Total Reduction Amount of Equation (4) (mm) Rt	Water Flow Density of Secondary Cooling Water for Second Intensity Section (L/m²/min) W1	Degree of Center Segregation (Cmax/C0)	Porosities	Internal Cracking
Invention Example 1	350	5.7	1.5	0.70	1.00	5.0	26.7	15.0	1.055	No	No
Invention Example 2			1.3	0.80	2.00	9.0	26.7	17.0	1.048	No	No
Invention Example 3			1.1	0.80	2.50	12.5	26.7	13.0	1.042	No	No
Comparative Example 1			1.0	0.70	0.05	0.6	26.7	14.0	1.125	Yes	No
Comparative Example 2			1.2	0.80	5.00	40.0	26.7	15.0	1.115	No	Yes
Comparative Example 3			0.9	0.80	1.00	4.0	26.7	5.0	1.128	Yes	No
Invention Example 4	400	5.0	0.8	0.70	1.50	7.0	37.2	20.0	1.052	No	No
Invention Example 5			0.7	0.60	2.50	8.0	37.2	12.0	1.045	No	No
Invention Example 6			0.8	0.70	1.00	4.0	37.2	14.0	1.057	No	No
Comparative Example 4			0.6	0.80	0.05	0.4	37.2	15.0	1.135	Yes	No
Comparative Example 5			0.6	0.80	7.00	24.0	37.2	15.0	1.104	No	Yes
Comparative Example 6			0.7	0.70	2.00	3.0	37.2	5.0	1.133	Yes	No
Invention Example 7	450	11	0.5	0.60	2.50	10.0	53.6	15.0	1.046	No	No
Invention Example 8			0.4	0.50	2.00	6.0	53.6	15.0	1.058	No	No
Invention Example 9			0.6	0.50	5.00	25.0	53.6	30.0	1.041	No	No
Comparative Example 7			0.5	0.50	0.20	6.0	53.6	29.0	1.134	Yes	No
Comparative Example 8			0.5	0.50	20.00	55.0	53.6	20.0	1.175	No	Yes
Comparative Example 9			0.4	0.60	5.00	15.0	53.6	3.0	1.235	Yes	No

The degree of center segregation of the slabs used for the evaluation in the test was measured in the following manner. In a cross section perpendicular to the slab withdrawal direction, a carbon concentration was analyzed at equal intervals in a thickness direction of each of the slabs. The maximum value in the thickness direction was designated as Cmax, and an analyzed carbon concentration of molten steel taken from a tundish during casting was designated as C0. Cmax/C0 was determined as the degree of center segregation. Regarding the degree of center segregation, values closer to 1.0 indicate lower degrees of center segregation of the slab. In the present invention, slabs having a degree of center segregation of 1.10 or greater were determined to be inferior in terms of the degree of center segregation.
The presence or absence of the porosities and internal cracking in the slabs was determined by performing microscopic observation on a thickness-wise central portion and a neighboring region of the slabs, in a cross section perpendicular to the slab withdrawal direction.
The casting speed of slab for each of the slabs was set such that the solid phase ratio of the central portion in the thickness direction of the slab in the soft reduction zone ranged from 0.1 to the limit of fraction solid for liquid flow. In Invention Examples 1 to 9, the reduction gradient and the water flow density of secondary cooling water were set to satisfy inequality (1), equation (2), and inequality (3), shown above. In Comparative Examples 1, 4, and 7, the reduction gradient was set at a value below the lower limit of an optimal range of the reduction gradient determined by inequality (1) and equation (2). In Comparative Examples 2, 5, and 8, the reduction gradient was set at a value above the upper limit of the optimal range of the reduction gradient determined by inequality (1) and equation (2). Furthermore, in Comparative Examples 2 and 5, the total reduction amount (Rt) was set at a value above the upper limit of inequality (4). Furthermore, in Comparative Examples 3, 6, and 9, the amount (W1) of secondary cooling water in the second intensity section was set at a value below the lower limit of inequality (3).
As is apparent from the degree of center segregation shown in Table 1, Invention Examples 1 to 9 all had a good degree of center segregation of less than 1.10. In addition, no porosities or internal cracking was observed in the slabs of Invention Examples 1 to 9.
Regarding Comparative Example 1, the optimal range of the reduction gradient determined by inequality (1) and equation (2) is 0.1 to 4.1 mm/m. In Comparative Example 1, the reduction gradient was set at 0.05 mm/m. In Comparative Example 1, the reduction gradient was insufficient, which resulted in a degree of center segregation of greater than 1.100. In addition, in Comparative Example 1, porosities were observed in the slab. In Comparative Examples 4 and 7, the reduction gradient was insufficient, which resulted in a center segregation of greater than 1.100. In addition, in Comparative Examples 4 and 7, porosities were observed in the slabs.
Regarding Comparative Example 2, the optimal range of the reduction gradient determined by inequality (1) and equation (2) is 0.1 to 3.5 mm/m. In Comparative Example 2, the reduction gradient was set at 5 mm/m. In Comparative Example 2, internal cracking occurred in the slab. Accordingly, it is believed that the total reduction amount of Comparative Example 2 was excessively large.
In Comparative Examples 5 and 8, the reduction gradient was excessive, which resulted in a center segregation of greater than 1.100. In Comparative Example 5, internal cracking occurred in the slab. Accordingly, it is believed that the total reduction amounts of Comparative Examples 5 and 8 was excessively large. In addition, in Comparative Examples 5 and 8, inverted V segregation, which occurs in the case of excessive reduction, was partially observed.
In Comparative Examples 3, 6, and 9, the center segregation was greater than 1.100. In Comparative Examples 3, 6, and 9, the solidified microstructures were coarse, which resulted in inferior degrees of center segregation and porosities compared with Invention Examples 1 to 9. It is believed that the reason for this is that, in Comparative Examples 3, 6, and 9, the water flow density for cooling water in the second intensity section was lower than that of Invention Examples 1 to 9.

Reference Signs List

100: continuous casting machine
10: molten steel
11: slab
11a: unsolidified layer
11b: solidified shell
40: slab support roll
44: soft reduction zone (soft reduction region)
45: roll segment

Claims

A method for manufacturing a slab having a thickness of 350 mm or greater and a ratio of a width to the thickness of 1.6 to 7.2, the method using a continuous casting machine, the method comprising:
a soft reduction region setting step of setting a soft reduction region in the continuous casting machine, the soft reduction region containing a starting point for performing soft reduction on the slab and an endpoint at which the soft reduction is terminated, the soft reduction region being set based on a condition of cooling water for cooling the slab, in the continuous casting machine, a casting speed of slab, and a solid phase ratio of the slab determined from a composition of the slab;

a reduction gradient setting step of setting a reduction gradient for the soft reduction region based on a thickness of the slab, a width of the slab, and the casting speed of slab;

a secondary cooling intensity setting step of setting a first intensity section and a second intensity section, which are sections for performing secondary cooling in the soft reduction region, based on the solid phase ratio of the slab, the second intensity section being a section in which the secondary cooling is performed with a higher intensity than in the first intensity section; and

a continuous casting step of performing continuous casting by using the set soft reduction region, reduction gradient, first intensity section, and second intensity section.
The method for manufacturing a slab according to Claim 1, wherein
in the soft reduction region setting step, the soft reduction region is set by selecting, as the starting point, a position at which the solid phase ratio of a central portion in a thickness direction of the slab reaches 0.1, and, as the endpoint, a position at which the solid phase ratio of the central portion in the thickness direction of the slab reaches a limit of fraction solid for liquid flow,

in the reduction gradient setting step, the reduction gradient for the soft reduction region is set to satisfy relationships of inequality (1) and equation (2), shown below, and

in the secondary cooling intensity setting step, a water flow density for the secondary cooling in the second intensity section is set to satisfy inequality (3): $0.05 / (V \times α) < Z < 2.0 / (V \times α)$ $α = (β \times (D / Do) + γ)$ $W 1 \geq 10$

where V is the casting speed of slab (m/min), α is a thickness coefficient (-), Z is the reduction gradient (mm/m), D is a thickness (mm) of the slab when the slab is directly under a casting mold, Do is a reference thickness (mm) of the slab when the slab is directly under the casting mold, W1 is the water flow density (L/m²/min), β and γ are coefficients determined by a width W2 (mm) of the slab 11, and β and γ are expressed as follows, depending on ranges of the width W2 of the slab:

when W2 ≤ 1300, β = -0.51, and γ = 1.54,

when 1300 < W2 ≤ 1700, β = -0.49, and γ = 1.56,

when 1700 < W2 ≤ 2100, β = -0.47, and γ = 1.58, and

when 2100 < W2, β = -0.45, and γ = 1.60.
The method for manufacturing a slab according to Claim 1 or 2, further comprising a total reduction amount setting step of setting a total reduction amount for the slab, based on the thickness of the slab, wherein
in the total reduction amount setting step, the total reduction amount is set to satisfy a relationship of inequality (4), shown below: $Rt < (D / Do) \times (10 / α)$

where Rt is the total reduction amount (mm) for the slab, D is a thickness (mm) of the slab when the slab is directly under a casting mold, Do is a reference thickness (mm) of the slab when the slab is directly under the casting mold, and α is a thickness coefficient (-).