EP4729207A1

EP4729207A1 - Method for assessing slab, slab manufacturing method, and steel sheet manufacturing method

Info

Publication number: EP4729207A1
Application number: EP24859169.5A
Authority: EP
Inventors: Tomoya Odagaki; Harutaka CHATANI; Hodaka Aoki; Tetsuo Mochida; Yohei Kaneko
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-08-31
Filing date: 2024-07-05
Publication date: 2026-04-22
Also published as: JPWO2025047124A1; JP7779401B2; WO2025047124A1; CN121620427A; TW202510981A; KR20260036562A

Abstract

There is provided a method for assessing a slab, which method can determine, based on component concentrations in molten steel manufactured by a continuous casting machine, the risk that a slab manufactured from the molten steel will split into two pieces.

A method for assessing a slab containing Ti, C, Si, and Mn as components and manufactured using a continuous casting machine includes a first step of determining the index of a liquid fraction of scale formed on the slab using component concentrations in molten steel poured into a mold of a continuous casting machine, and a second step of determining that the slab is at risk of splitting into two pieces when the index of the liquid fraction of the scale is greater than or equal to a predetermined threshold value.

Description

Technical Field

The present invention relates to a method for assessing a slab, which method determines the risk of splitting into two pieces, a method for manufacturing a slab using the method, and a method for manufacturing a steel sheet.

Background Art

In recent years, in the field of automobiles, components, such as Si, Mn, and Ti, have been added in large amounts in order to reduce the weight of vehicle bodies and improve workability. Steels containing large amounts of the components, such as Si, Mn, and Ti, frequently suffer from the problem of splitting into two pieces during rolling, leading to a significant reduction in productivity. Here, splitting into two pieces refers to a phenomenon in which a crack forms in the central portion of the thickness of a steel sheet, causing a hot-rolled steel sheet to split into two pieces.
Even if splitting into two pieces is not apparent at the hot-rolled steel sheet stage, when the hot-rolled steel sheet is shipped with an internal crack, the splitting into two pieces may occur during pressing or bending. For these reasons, it is necessary to eliminate defects that cause splitting into two pieces.
The cause of splitting into two pieces is believed to be the oxidation of cross-sectional defects in slabs (cross-sectional cracks present on cut surfaces or voids in the last-solidified portions) in a heating furnace. However, not all slabs with cross-sectional defects split into two pieces, and the occurrence rate of splitting into two pieces varies greatly depending on the steel grade.
As a technique for inhibiting splitting into two pieces, Patent Literature 1 discloses a continuous casting method for steel, in which a slab is forcibly reduced at the final stage of solidification to close voids in the last-solidified portion. Patent Literature 2 discloses a method in which the secondary cooling flow rate is reduced to inhibit the confinement of a residual liquid phase even in a non-steady portion.

Citation List

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2003-94154
PTL 2: Japanese Unexamined Patent Application Publication No. 2003-334651

Summary of Invention

Technical Problem

In the related art, it has not been possible to explain differences in the occurrence of splitting into two pieces among steel grades, which has led to the following problems. When the method disclosed in Patent Literature 1 is used, it is possible to eliminate the cross-sectional defects of the slab. However, when the slab is forcibly reduced, the reaction force increases significantly, which greatly reduces the service life of the rolls and segments. Therefore, if this method is used for all steel grades, it will result in an economic problem of increased slab manufacturing costs.
To affect the solidified shell thickness using the method disclosed in Patent Literature 2, it is necessary to reduce the amount of cooling water from an early stage of solidification. However, weakened cooling increases bulging between rolls, leading to the risk of causing other defects, such as internal cracks. Therefore, the use of this method for all steel grades may increase the potential for unnecessary risks.
The present invention has been made in view of the above circumstances, and aims to provide a method for assessing a slab, which method can determine, based on component concentrations in molten steel manufactured by a continuous casting machine, the risk that a slab manufactured from the molten steel will split into two pieces.

Solution to Problem

Means to solve the above-mentioned problems are described below.

[1] A method for assessing a slab containing Ti, C, Si, and Mn as components and manufactured using a continuous casting machine includes a first step of determining the index of a liquid fraction of scale formed on a slab using component concentrations in molten steel poured into a mold of a continuous casting machine, and a second step of determining that the slab is at risk of splitting into two pieces when the index of the liquid fraction of the scale is greater than or equal to a predetermined threshold value.
[2] In the method for assessing a slab described in [1], the index of the liquid fraction of the scale is an index determined by a Ti concentration, a C concentration, a Si concentration, and a Mn concentration in the molten steel.
[3] In the method for assessing a slab described in [1] or [2], the index of the liquid fraction of the scale is an index determined by the following formula (1): f = A_Ti1 × [Ti] × {A_C × [C] + A_Si × [Si] + A_Mn × [Mn] + A_Ti2 × [Ti] + A_Si/Mn × ([Si]/[Mn])^0.5} where in the above formula (1), f is an index (-) of the liquid fraction of the scale, [Ti] is the Ti concentration (mass%) in the molten steel, [C] is the C concentration (mass%) in the molten steel, [Si] is the Si concentration (mass%) in the molten steel, [Mn] is the Mn concentration (mass%) in the molten steel, and A_Ti1, A_C, A_Si, A_Mn, A_Ti2, and A_Si/Mn are parameters.
[4] In the method for assessing a slab described in any one of [1] to [3], in the second step, when the index of the liquid fraction of the scale is less than the predetermined threshold value, the slab is determined not to be at risk of splitting into two pieces.
[5] A method for manufacturing a slab using a continuous casting machine includes changing a roll gap between reduction rolls of the continuous casting machine within a range in which a solid fraction at the center of the width and thickness of a strand in the casting direction is more than 0.0 and 1.0 or less when a slab is determined to be at risk of splitting into two pieces by the method for assessing a slab described in any one of [1] to [3]; and subjecting the strand to soft reduction.
[6] A method for manufacturing a slab using a continuous casting machine includes determining the presence or absence of a cross-sectional defect by measuring a temperature difference in the width direction of a slab in a range from a position at which a strand support roll is not provided to a position at which the slab is cut when the slab is determined to be at risk of splitting into two pieces by the method for assessing a slab described in any one of [1] to [3].
[7] A method for manufacturing a slab using a continuous casting machine includes capturing an image of a cut surface of a slab and generating image data, and determining the presence or absence of a cross-sectional defect using the image data when the slab is determined to be at risk of splitting into two pieces by the method for assessing a slab described in any one of [1] to [3].
[8] In the method for manufacturing a slab described in [7], the image of the cut surface is captured using an image-capturing device including an IR-cut filter.
[9] In the method for manufacturing a slab described in any one of [6] to [8], for the slab determined to have the cross-sectional defect, a void in the cross-sectional defect is welded.
[10] A method for manufacturing a steel sheet by heating and hot-rolling a slab includes heating and hot-rolling a slab that has been determined not to be at risk of splitting into two pieces by the method for assessing a slab described in [4].
[11] A method for manufacturing a steel sheet by heating and hot-rolling a slab includes:
heating and hot-rolling a slab that has been manufactured by the method for manufacturing a slab described in [5].
[12] A method for manufacturing a steel sheet by heating and hot-rolling a slab includes heating and hot-rolling a slab that has been determined to be free of the cross-sectional defect by the method for manufacturing a slab described in any one of [6] to [8].
[13] A method for manufacturing a steel sheet by heating and hot-rolling a slab includes heating and hot-rolling a slab that has been manufactured by the method for manufacturing a slab described in [9].

Advantageous Effects of Invention

According to the present invention, by using the index of the liquid fraction of the scale determined from the component concentrations in the molten steel, it is possible to determine the risk that the slab manufactured from the molten steel will split into two pieces. In this way, the risk of splitting into two pieces is determined. Thus, a slab determined to be at risk of splitting into two pieces can be selected and handled. This makes it possible to inhibit an increase in slab manufacturing cost and an increase in potential for unnecessary risk.

Brief Description of Drawings

[Fig. 1] Fig. 1 is a schematic side view of a continuous casting machine that can perform a method for assessing a slab and a method for manufacturing a slab according to the present embodiment.
[Fig. 2] Fig. 2 is a graph illustrating the relationship between the index f of the liquid fraction of scale and the occurrence rate of splitting into two pieces.
[Fig. 3] Fig. 3 is a graph illustrating the relationship between the temperature difference in the width direction and the cross-sectional defect of the slab. Description of Embodiments

The present invention will be described in detail below with reference to embodiments of the present invention. The following embodiments are preferred examples of the present invention, and the present invention is not limited to these embodiments.
Fig. 1 is a schematic side view of a continuous casting machine 100 that can perform a method for assessing a slab and a method for manufacturing a slab according to the present embodiment. The continuous casting machine 100 includes a mold 10, a tundish 12 installed above the mold 10, and multiple pairs of strand support rolls 32 arranged side by side below the mold 10. A sliding nozzle 14 for adjusting the flow rate of molten steel 18 is disposed at the bottom portion of the tundish 12. A submerged entry nozzle 16 is disposed on the underside of the sliding nozzle 14.
Molten steel 18 is poured into the mold 10 through the submerged entry nozzle 16. The molten steel 18 poured into the mold 10 solidifies by heat transfer through the inner surface of the mold 10, thereby forming a solidified shell 20. This forms a strand 24 with the solidified shell 20 as the outer shell and an inner liquid core 22 made of the molten steel 18.
The multiple pairs of strand support rolls 32 including support rolls 26, guide rolls 28, and pinch rolls 30 are disposed below the mold 10. The pinch rolls 30 support the strand 24 and also serve as drive rolls for withdrawing the strand 24. Spray nozzles, such as water spray nozzles or air-mist spray nozzles (not illustrated), are disposed in the gaps between adjacent strand support rolls arranged in the casting direction, thereby forming a secondary cooling zone. The strand 24 is cooled by cooling water (hereinafter, also referred to as "secondary cooling water") sprayed from the spray nozzles in the secondary cooling zone while the strand is withdrawn. This reduces the inner liquid core 22 and allows the solidified shell 20 to grow. Thereafter, the strand 24 is appropriately cooled to allow the solidification of the liquid core 22 to proceed, resulting in the complete solidification of the strand 24. Multiple conveyor rollers 34 configured to convey the cast strand 24 are disposed downstream of the strand support rolls 32. A strand cutter 36 configured to cut the cast strand 24 into slabs 38 of a predetermined length is disposed above the conveyor rollers 34.
A soft reduction zone 42 where the roll gap between the opposing guide rolls 28 can be adjusted is provided upstream of the solidification completion position 40 of the strand 24 in the casting direction. The guide rolls constituting the soft reduction zone 42 are configured in such a manner that the roll gap can be adjusted. By adjusting the roll gap, it is possible to apply soft reduction to the strand 24, or it is also possible not to perform soft reduction. In the present embodiment, the soft reduction zone 42 is provided in such a manner that the strand 24 having a solid fraction at the center of the thickness and width of the strand 24 of at least 0.1 to 1.0 is positioned within the installation range of the soft reduction zone 42. The solid fraction at the center of the thickness of the strand 24 is a solid fraction at the center of the thickness of the strand excluding the end portions in the width direction of the strand, but may be represented by the solid fraction at the center of the width and thickness of the strand. Here, the solid fraction is an index indicating the progress of solidification. The solid fraction is expressed in the range of 0.0 to 1.0. A solid fraction of 0.0 (zero) indicates that the material is not yet solidified. A solid fraction of 1.0 indicates that the material is completely solidified.
The reduction gradient in the soft reduction zone 42 is expressed as a reduction in the roll gap per meter in the casting direction (mm/m). The reduction rate (mm/min) of the strand 24 in the soft reduction zone 42 is determined by the product of the reduction gradient (mm/m) and the casting speed (m/min). In the soft reduction zone 42, the strand 24 is reduced at a reduction rate of 0.2 mm/min to 1.0 mm/min. Spray nozzles for cooling the strand 24 are also arranged between the strand support rolls included in the soft reduction zone 42. Although Fig. 1 illustrates an example in which only the guide rolls 28 are disposed in the soft reduction zone 42, the pinch rolls 30 may also be disposed in the soft reduction zone 42. The strand support rolls 32 arranged in the soft reduction zone 42 are also referred to as "reduction rolls".
A thermal camera 44 for measuring the surface temperature of the strand 24 in the width direction is disposed in the range from a position where the strand support rolls 32 are not provided to a position where the strand cutter 36 is provided in the casting direction. The thermal camera 44 measures the temperature difference in the width direction of the strand 24. Because the thermal camera 44 is disposed in the range from the position where the strand support rolls 32 are not provided to the position where the strand cutter 36 is provided, the surface temperature of the strand 24 can be measured without being affected by water vapor and steam. This makes it possible to easily measure the temperature difference in the width direction of the strand 24. Furthermore, an illumination device 46 is provided to illuminate a cut surface of a slab 38 cut by the strand cutter 36, and an image-capturing device 48 is provided to capture an image of the cut surface and generate image data.
In the continuous casting machine 100, if the components in the slab 38 manufactured without soft reduction in the soft reduction zone 42 include Ti, C, Si, and Mn, splitting into two pieces may occur in the hot-rolled steel sheet manufactured from the slab 38. Splitting into two pieces is a phenomenon in which a steel sheet splits into two pieces due to a crack in the center of the thickness during rolling or at the customer's site.
The inventors have conducted intensive studies on the cause of the occurrence of splitting into two pieces in a hot-rolled steel sheet manufactured by hot-rolling the slab 38 containing Ti, C, Si, and Mn as its components and have found that splitting into two pieces is closely related to the presence of a cross-sectional defect in the slab 38 and a high liquid fraction of scale formed in the high-temperature environment inside a heating furnace. Therefore, even if a cross-sectional defect is present in the slab 38, the hot-rolled steel sheet will not split into two pieces when the liquid fraction of the scale formed in the high-temperature environment inside the heating furnace is low.
The index of the liquid fraction of the scale formed in the high-temperature environment inside the heating furnace can be determined using the component concentrations in the molten steel 18, that is, the Ti concentration, the C concentration, the Si concentration, and the Mn concentration. Therefore, in the method for assessing a slab according to the present embodiment, a first step is performed in which the index of the liquid fraction of the scale is determined using the component concentrations in the molten steel 18 poured into the mold 10, that is, the Ti concentration, the C concentration, the Si concentration, and the Mn concentration. If the index of the liquid fraction of the scale obtained in the first step is greater than or equal to a predetermined threshold value, a second step is performed in which the slab 38 is determined to be at risk of splitting into two pieces. Specifically, in the first step, the index f of the liquid fraction of the scale is calculated using the Ti concentration, C concentration, Si concentration, and Mn concentration of the molten steel 18 and the following formula (1). f = A_Ti1 × [Ti] × {A_C × [C]+A_Si × [Si] + A_Mn × [Mn] + A_Ti2 × [Ti] + A_Si/Mn × ([Si]/[Mn])^0.5}
In the above formula (1), f is the index (-) of the liquid fraction of the scale. [Ti] is the Ti concentration (mass%) of the molten steel 18. [C] is the C concentration (mass%) of the molten steel 18. [Si] is the Si concentration (mass%) of the molten steel 18. [Mn] is the Mn concentration (mass%) of the molten steel 18. A_Ti1, A_C, A_Si, A_Mn, A_Ti2, and A_Si/Mn are parameters. (-) indicates dimensionless. For example, when the temperature in the heating furnace is 1,200°C, the parameters of the above formula (1) are A_Ti1 = 8.2, A_C = -0.7, A_Si = 2.0, A_Mn = -0.5, A_Ti2 = 23, and A_Si/Mn = 2.7.
In the second step, a slab for which the index f of the liquid fraction of scale determined using the above formula (1) is greater than or equal to a predetermined threshold value is determined to be at risk of splitting into two pieces. In contrast, a slab for which the index f of the liquid fraction of scale is less than a predetermined threshold value may be determined not to be at risk of splitting into two pieces.
Fig. 2 is a graph illustrating the relationship between the index f of the liquid fraction of scale and the occurrence rate of splitting into two pieces. The horizontal axis of Fig. 2 is the index f (-) of the liquid fraction of scale. The index f is a value calculated using the above formula (1) and the Ti concentration, C concentration, Si concentration, and Mn concentration in the molten steel 18. The parameters used in formula (1) were A_Ti1: 8.2, A_C: -0.7, A_Si: 2.0, A_Mn: -0.5, A_Ti2: 23, and A_Si/Mn: 2.7.
The vertical axis in Fig. 2 is the occurrence rate of splitting into two pieces (%). The occurrence rate of splitting into two pieces is the occurrence rate of splitting into two pieces (%) that occurred in a hot-rolled steel sheet manufactured by hot-rolling a slab that was cast without performing soft reduction using the molten steel 18 having component concentrations from which the index f of the liquid fraction of scale was calculated. As illustrated in Fig. 2, when the index f of the liquid fraction of scale was 1.00 or more, the occurrence rate of splitting into two pieces in the hot-rolled steel sheet manufactured by hot-rolling the slab was 0.2% or more. In contrast, when a slab had an index f of the liquid fraction of scale of less than 1.00, the occurrence rate of splitting into two pieces in a hot-rolled steel sheet manufactured by hot-rolling the slab was 0%. From the results, the index f of the liquid fraction of scale is calculated using the component concentrations in the molten steel 18, that is, the Ti concentration, C concentration, Si concentration, Mn concentration, and formula (1) described above, and it is determined whether the index f of the risk of splitting into two pieces is 1.00 or more. This makes it possible to determine the risk that the slab manufactured from the molten steel will split into two pieces.
When the risk of splitting into two pieces can be determined in this way, it is possible to select and handle a slab determined to be at risk of splitting into two pieces, thereby inhibiting an increase in the manufacturing cost of the slab 38 and an increase in unnecessary risk due to handling the slab 38 that is not at risk of splitting into two pieces. The value 1.00 used to determine the risk of splitting into two pieces is an example of a threshold value for the index f of the liquid fraction of scale. The threshold value of the index f of the liquid fraction of scale can be determined in advance by investigating the relationship between the index f of the liquid fraction of scale and the occurrence rate of splitting into two pieces, as illustrated in Fig. 2.
If the slab is determined to be at risk of splitting into two pieces by the method for assessing a slab according to the present embodiment, when a cross-sectional defect is present in the slab, there is a high possibility that splitting into two pieces will occur in a hot-rolled steel sheet manufactured by hot rolling. For this reason, in a method for manufacturing a slab according to the present embodiment, for a slab determined to be at risk of splitting into two pieces by the above-described method for assessing a slab, the continuous casting conditions are changed in such a manner that no cross-sectional defect occurs in the slab.
For example, preferably, the roll gap of the reduction rolls in the soft reduction zone 42 in the continuous casting machine 100 is decreased to be smaller than the thickness of the strand 24, and soft reduction is performed at a reduction rate of 0.2 mm/min or more and 1.0 mm/min or less, thereby inhibiting a cross-sectional defect in the slab to be manufactured. This soft reduction is preferably performed in such a manner that the solid fraction at the center of the width and thickness of the strand 24 is in the range of more than 0.0 and 1.0 or less in the casting direction. The soft reduction may be performed in such a manner that the solid fraction is in the range of more than 0.0 and 0.7 or less (critical solid fraction for flow). This makes it possible to inhibit the occurrence of a cross-sectional defect in the slab 38.
In this way, when a slab 38 free of any cross-sectional defects is manufactured by changing the continuous casting conditions, even if scale with a high liquid fraction occurs in the high-temperature environment inside the heating furnace, splitting into two pieces can be inhibited in a hot-rolled steel sheet manufactured by hot-rolling the slab.
When a slab is determined not to be at risk of splitting into two pieces by the method for assessing a slab according to the present embodiment, even if there is a cross-sectional defect in the slab, splitting into two pieces will not occur in the hot-rolled steel sheet manufactured by hot-rolling the slab. Therefore, for a slab determined not to be at risk of splitting into two pieces by the method for assessing a slab, it is preferable to change the continuous casting conditions in such a manner that the roll gap of the guide rolls 28 in the soft reduction zone 42 is increased to the thickness of the strand 24, thereby avoiding the application of soft reduction to the strand 24. This inhibits a decrease in the service life of the guide rolls and roll segments in the soft reduction zone 42, thereby inhibiting an increase in the manufacturing cost of the slab 38.
As described above, when a hot-rolled steel sheet is produced from a slab manufactured by the method for manufacturing a slab according to the present embodiment, splitting into two pieces does not occur in the steel sheet. Therefore, when the slab is heated in a heating furnace and hot-rolled in a rolling mill, it is possible to manufacture a hot-rolled steel sheet that does not split into two pieces during rolling or at the customer's site.
Formula (1) used to determine the index f of the liquid fraction of scale will be described below. When a slab has a high liquid fraction of scale in a high-temperature environment inside a heating furnace, the occurrence rate of splitting into two pieces increases in a hot-rolled steel sheet manufactured by hot-rolling the slab. The oxygen diffusion rate in liquid scale is higher than that in solid scale, thus resulting in sufficient diffusion and oxidation within the cross-sectional defect. This liquid scale is elongated during hot rolling, causing splitting into two pieces during rolling of the hot-rolled steel sheet or at the customer's site. Furthermore, the investigations by the inventors have revealed that the liquid fraction of scale is particularly high when fayalite (Fe₂SiO₄) is formed, and that the liquid fraction of scale increases further with the addition of Ti.
Formula (1) for determining the index f of the liquid fraction of scale is a regression equation created in consideration of the degrees to which the component concentrations in the molten steel 18 affect the liquid fraction of the scale. The regression equation was first established by calculating the liquid fraction of scale in a predetermined high-temperature environment inside a heating furnace using thermodynamic calculations. Considering the dependence of the liquid fraction on the components, the base regression equation and its parameters were determined. In the scale of an FeO-SiO₂-MnO-TiO₂ system, the melting point decreases as the composition approaches that of fayalite (Fe₂SiO₄). Therefore, a higher Si concentration in the molten steel 18 is more likely to form scale with a low melting point. Larger amounts of C and Mn, which are additive elements to be oxidized, result in the formation of SiO₂ less readily. Therefore, the parameters for the C and Mn concentrations in the regression equation were set to negative values. When the amount of Mn added is large, depending on its ratio to Si, scale formed may shift to being mainly composed of MnO. To take this into consideration, the term Si concentration/Mn concentration was introduced into the regression equation. Ti has the effect of lowering the melting point of fayalite. For this reason, the Ti concentration was introduced into the regression equation as a factor multiplying all terms.
The regression equation and each parameter were determined based on these considerations, and then the results were compared with the actual occurrence rate of splitting into two pieces in steel sheets manufactured by heating to 1,200°C in a heating furnace and hot rolling. The boundary between the index f of the liquid fraction of scale in which splitting into two pieces did not occur and the index f of the liquid fraction of scale in which splitting into two pieces occurred was normalized to 1.00. An example of the regression equation determined in this manner is formula (1) described above. Examples of the parameters at that time are as follows: A_Ti1: 8.2, A_C: -0.7, A_Si: 2.0, A_Mn: -0.5, A_Ti2: 23, and A_Si/Mn: 2.7.
In the above example, the regression equation for calculating the index f of the liquid fraction of scale and each parameter are determined by calculating the liquid fraction of scale using thermodynamic calculations. However, a method for determining the regression equation for calculating the index f of the liquid fraction of scale and each parameter is not limited thereto. For example, scale may be actually produced in a laboratory experiment, and the state of the scale may be investigated to determine the regression equation and each parameter. Furthermore, the regression equation and each parameter may be determined from the relationship between the occurrence rate of two-piece splitting defects in an actual machine and the component concentrations in the molten steel 18.
In the method for assessing a slab according to the present embodiment, an example has been given in which the risk that a slab to be manufactured by the continuous casting machine 100 will split into two pieces is determined using the component concentrations of the molten steel 18 poured into the mold 10 of the continuous casting machine 100; however, the method is not limited thereto. The method for assessing a slab according to the present embodiment can determine the risk that not only slabs to be manufactured by the continuous casting machine 100 but also slabs manufactured in the past will split into two pieces. In this case, instead of the component concentrations of the molten steel 18 poured into the mold 10, the component concentrations of a previously manufactured slab may be used to determine an index of the liquid fraction of scale that will form in the slab in the high-temperature environment inside the heating furnace.
The embodiment of the present invention is not limited to the above-described embodiment, and various changes can be made. In the method for manufacturing a slab according to the present embodiment, an example has been given in which for a slab 38 determined to be at risk of splitting into two pieces in the second step, the continuous casting conditions are changed, and soft reduction is performed; however, the method is not limited thereto. For the slab 38 determined to be at risk of splitting into two pieces in the second step, the presence or absence of a cross-sectional defect in the slab 38 is determined, and the cross-sectional defect of the slab 38 determined to have the cross-sectional defect may be welded. The presence or absence of a cross-sectional defect in the slab 38 can be determined by the following two methods for determining a cross-sectional defect.

First, a method 1 for determining a cross-sectional defect in a slab will be described. A large temperature difference in the width direction of the strand 24 at the final stage of solidification is likely to cause a cross-sectional defect in the slab 38 manufactured by cutting the strand 24. The temperature difference in the width direction of the strand 24 is measured in the range from the position where the strand support rolls 32 are not provided to the position where the strand cutter 36 is provided in the casting direction.
Fig. 3 is a graph illustrating the relationship between the temperature difference in the width direction and the cross-sectional defects of the slabs. In Fig. 3, the horizontal axis represents the width dimension (mm) of the strand, and the vertical axis represents the temperature difference (°C) in the width direction. The temperature difference in the width direction is the temperature difference between the maximum temperature within a range of ±100 mm from the center of the width direction of the strand 24 and the maximum temperature across the entire width direction of the strand (excluding the range of ±100 mm from the center of the width direction). In the graph, ∘ marks denote manufacturing examples of the slabs 38 without cross-sectional defects, and × marks denote manufacturing examples of the slabs 38 with cross-sectional defects.
As illustrated in Fig. 3, when the temperature difference in the width direction of the strand 24 was 80°C or more, cross-sectional defects were likely to occur in the slab 38. When the temperature difference in the width direction of the strand 24 was 100°C or more, cross-sectional defects occurred in the slab 38. From these results, it can be seen that the presence or absence of cross-sectional defects in the slab 38 can be determined by using the temperature difference in the width direction of the strand 24 in the range from the completion of solidification of the strand 24 to the position where the strand cutter 36 is provided.
Specifically, if the slab is determined to be at risk of splitting into two pieces in the second step, the temperature difference in the width direction of the strand 24 is measured using a thermal camera 44. When the temperature difference is greater than 80°C, the slab 38 to be manufactured is determined to have a cross-sectional defect. When the temperature difference is 80°C or less, the slab 38 to be manufactured is determined to be free of any cross-sectional defects.
The temperature difference of 80°C is an example of a predetermined threshold value of the temperature difference, and is determined in advance by investigating the relationship between the temperature difference in the width direction and the presence or absence of the cross-sectional defect, as illustrated in the graph of Fig. 3. In this way, in the method for manufacturing a slab according to the present embodiment, for a slab determined to be at risk of splitting into two pieces in the second step, the presence or absence of the cross-sectional defect may be determined by the method 1 for determining cross-sectional defects.

A method 2 for determining a cross-sectional defect in a slab will be described below. In the method 2 for determining a cross-sectional defect, the presence or absence of a cross-sectional defect in the slab 38 is determined by producing a cut surface with the strand cutter 36 and then using image data obtained by capturing an image of the cut surface with an image-capturing device. When a cross-sectional defect occurs in the slab 38, a crack (tear) is formed on the cut surface of the slab 38. Therefore, the presence or absence of a cross-sectional defect in the slab 38 can be determined by detecting cracks on the cut surface using image data generated by capturing an image of the cut surface of slab 38.
The detection of cracks may be performed through visual inspection of the image data by an operator, or cracks may be detected by image processing of the image data. Cracks have a lower brightness than other portions. Therefore, for example, the image data is binarized using a brightness threshold value that can distinguish between cracks and other portions, and the width and height dimensions of the cracks are determined. The presence or absence of cracks can be detected by comparing these dimensions with the previously recorded crack dimensions.
The image-capturing device 48 preferably includes an IR-cut filter. The cut surface of the slab 38 is red hot. Therefore, if an image of the cut surface is directly captured, the difference in brightness between pixels at the crack and pixels at the other portions is small, making it difficult to detect the cracks. In contrast, the use of the image-capturing device equipped with the IR-cut filter that cuts out light with wavelengths of 700 nm or more can reduce the effect of red heat, thereby enabling stable detection of cracks on the cut surface.
In addition, the illumination device 46 is preferably used to illuminate the cut surface of the slab 38. Furthermore, the illumination device 46 preferably illuminates the cut surface of the slab 38 from either above or below at an angle relative to the thickness direction of the slab 38. Thus, when the cut surface is illuminated at an angle by the illumination device 46, shadows are formed in the cracks, thereby enabling more stable detection of the cracks.
When a cross-sectional defect is present in a slab determined to be at risk of splitting into two pieces in the second step, there is a high possibility that splitting into two pieces will occur in the hot-rolled steel sheet manufactured by hot rolling. Therefore, if a cross-sectional defect is determined to be present by the method 1 or 2 for determining a cross-sectional defect, the crack formed on the cut surface of the slab 38 is welded to close voids. This makes it possible to inhibit the entry of scale with a high liquid fraction into the voids of the cross-sectional defect. Accordingly, even in a slab that has been determined to have a cross-sectional defect and presents the risk of splitting into two pieces, the occurrence of splitting into two pieces in the hot-rolled steel sheet produced by hot rolling can be inhibited.
When it is determined by the method 1 or 2 for determining a cross-sectional defect that there are no cross-sectional defects, the hot-rolled steel sheet manufactured from the slab 38 does not split into two pieces. Accordingly, the slab 38 determined by the method 1 or 2 for determining a cross-sectional defect to be free of any cross-sectional defects may be used as is in the manufacture of a hot-rolled steel sheet. Thus, when the determination is made by the method 1 or 2 for determining a cross-sectional defect, the number of slabs to be removed from the production line for welding the cut surface of the slab 38 can be reduced, thereby inhibiting an increase in slab manufacturing costs.

EXAMPLES

Hereinafter, examples are described to demonstrate the effects of the present invention. Slabs were produced using continuous casting machine 100 from steel grades A to C. The compositions of steel grades A to C, the index f of the liquid fraction of scale, whether soft reduction was performed, and the occurrence rate of splitting into two pieces are presented in Table 1 below. The index f of the liquid fraction of scale was calculated using the following formula (2).

f = 8.2 × [Ti] × {-0.7 × [C] + 2.0 × [Si] - 0.5 × [Mn] + 23 × [Ti] + 2.7 × ([Si]/[Mn])^0.5}

[Table 1]

No	Steel grade	C (mass%)	Si (mass%)	Mn (mass%)	Ti (mass%)	Si/Mn (-)	f (-)	Soft reduction	Occurrence rate of splitting into two pieces (%)
1	A	0.080	0.65	1.20	0.033	0.54	0.92	-	0.0
2	A	0.080	0.65	1.10	0.035	0.59	1.03	-	0.3
3	A	0.070	0.61	1.25	0.032	0.49	0.83	-	0.0
4	A	0.065	0.68	1.15	0.035	0.59	1.04	performed	0.0
5	B	0.170	1.20	2.20	0.030	0.55	0.95	-	0.0
6	B	0.165	1.30	2.00	0.031	0.65	1.11	-	0.5
7	B	0.175	1.10	2.30	0.028	0.48	0.79	-	0.0
8	B	0.172	1.25	2.10	0.032	0.60	1.09	performed	0.0
9	C	0.090	0.70	1.45	0.100	0.48	3.93	performed	0.0

In Table 1, multiple tests on the same steel grade were conducted to assess variations in chemical composition. In Table 1, the occurrence rate of splitting into two pieces indicates the percentage (%) at which splitting into two pieces occurred in the hot-rolled steel sheets manufactured from the respective slabs.
Test Nos. 1 to 3 and 5 to 7 indicate the validity of the determination based on the index f of the liquid fraction of scale due to component variations. As presented in Table 1, when slabs having an index f of the liquid fraction of scale of 1.00 or more were used, splitting into two pieces occurred in hot-rolled steel sheets manufactured by subjecting the slabs to heating to 1,200°C in a heating furnace and then hot rolling. In contrast, when slabs having an index f of the liquid fraction of scale of less than 1.00 were used, splitting into two pieces did not occur in hot-rolled steel sheets manufactured by subjecting the slabs to heating to 1,200°C in the heating furnace and then hot rolling. Based on the results, the indices f of the liquid fractions of the scales of the slabs manufactured from molten steels are calculated using the component concentrations of the molten steels before they are cast in the continuous casting machine. It has been confirmed that by determining whether the calculated index f is greater than or equal to a predetermined threshold value, the risk that the hot-rolled steel sheet manufactured by hot-rolling the slab will split into two pieces can be assessed.
Test Nos. 4, 8, and 9 indicate the results of performing soft reduction under changed casting conditions for slabs that were determined to be at risk of splitting into two pieces and that had an index f of the liquid fraction of scale of 1.00 or more. In Test Nos. 4, 8, and 9, soft reduction was performed to eliminate cross-sectional defects in the slabs that were determined to be at risk of splitting into two pieces. The reduction rates of the soft reduction applied were No. 4: 0.24 mm/min, No. 8: 0.66 mm/min, and No. 9: 0.96 mm/min. Regarding the slabs of No. 4, 8, and 9, which were subjected to soft reduction, splitting into two pieces did not occur in hot-rolled steel sheets manufactured by subjecting the slabs to heating to 1,200°C in the heating furnace and hot rolling. These results revealed that with regard to the slabs determined to be at risk of splitting into two pieces, when appropriate soft reduction is performed at the final stage of solidification, splitting into two pieces in the hot-rolled steel sheets manufactured by hot-rolling the slabs can be inhibited.

Reference Signs List

10 mold
12 tundish
14 sliding nozzle
16 submerged entry nozzle
18 molten steel
20 solidified shell
22 liquid core
24 strand
26 support roll
28 guide roll
30 pinch roll
32 strand support roll
34 conveyor roller
36 strand cutter
38 slab
40 solidification completion position
42 soft reduction zone
44 thermal camera
46 illumination device
48 image-capturing device
100 continuous casting machine

Claims

A method for assessing a slab containing Ti, C, Si, and Mn as components and manufactured using a continuous casting machine, the method comprising:
a first step of determining an index of a liquid fraction of scale formed on a slab using concentrations of components in molten steel poured into a mold of a continuous casting machine; and

a second step of determining that the slab is at risk of splitting into two pieces when the index of the liquid fraction of the scale is greater than or equal to a predetermined threshold value.
The method for assessing a slab according to claim 1, wherein the index of the liquid fraction of the scale is an index determined by a Ti concentration, a C concentration, a Si concentration, and a Mn concentration in the molten steel.
The method for assessing a slab according to claim 1 or 2, wherein the index of the liquid fraction of the scale is an index determined by the following formula (1): f = A_Ti1 × [Ti] × {A_C × [C] + A_Si × [Si] + A_Mn × [Mn] + A_Ti2 × [Ti] + A_Si/Mn × ([Si]/[Mn])^0.5} where in the above formula (1), f is an index (-) of the liquid fraction of the scale, [Ti] is the Ti concentration (mass%) in the molten steel, [C] is the C concentration (mass%) in the molten steel, [Si] is the Si concentration (mass%) in the molten steel, [Mn] is the Mn concentration (mass%) in the molten steel, and A_Ti1, A_C, A_Si, A_Mn, A_Ti2, and A_Si/Mn are parameters.
The method for assessing a slab according to any one of claims 1 to 3, wherein in the second step, when the index of the liquid fraction of the scale is less than the predetermined threshold value, the slab is determined not to be at risk of splitting into two pieces.
A method for manufacturing a slab using a continuous casting machine, comprising:
changing a roll gap between reduction rolls of a continuous casting machine within a range in which a solid fraction at a center of a width and thickness of a strand in a casting direction is more than 0.0 and 1.0 or less when a slab is determined to be at risk of splitting into two pieces by the method for assessing a slab according to any one of claims 1 to 3; and subjecting the strand to soft reduction.
A method for manufacturing a slab using a continuous casting machine, comprising:
determining presence or absence of a cross-sectional defect by measuring a temperature difference in a width direction of a slab in a range from a position at which a strand support roll is not provided to a position at which the slab is cut when the slab is determined to be at risk of splitting into two pieces by the method for assessing a slab according to any one of claims 1 to 3.
A method for manufacturing a slab using a continuous casting machine, comprising:
capturing an image of a cut surface of a slab and generating image data, and determining presence or absence of a cross-sectional defect using the image data when the slab is determined to be at risk of splitting into two pieces by the method for assessing a slab according to any one of claims 1 to 3.
The method for manufacturing a slab according to claim 7, wherein the image of the cut surface is captured using an image-capturing device including an IR-cut filter.
The method for manufacturing a slab according to any one of claims 6 to 8, wherein, for the slab determined to have the cross-sectional defect, a void in the cross-sectional defect is welded.
A method for manufacturing a steel sheet by heating and hot-rolling a slab, comprising:
heating and hot-rolling a slab that has been determined not to be at risk of splitting into two pieces by the method for assessing a slab according to claim 4.
A method for manufacturing a steel sheet by heating and hot-rolling a slab, comprising:
heating and hot-rolling a slab that has been manufactured by the method for manufacturing a slab according to claim 5.
A method for manufacturing a steel sheet by heating and hot-rolling a slab, comprising:
heating and hot-rolling a slab that has been determined to be free of the cross-sectional defect by the method for manufacturing a slab according to any one of claims 6 to 8.
A method for manufacturing a steel sheet by heating and hot-rolling a slab, comprising:
heating and hot-rolling a slab that has been manufactured by the method for manufacturing a slab according to claim 9.