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

Abstract

To provide a method for assessing a slab with which it is possible to assess, from the component concentration of molten steel manufactured by a continuous casting apparatus, the risk of a slab manufactured using the molten steel cracking into two pieces.　This method for assessing a slab that contains Ti, C, Si, and Mn as components and that is manufactured using a continuous casting apparatus comprises: a first step for obtaining, using the component concentration of molten steel injected into the mold of the continuous casting apparatus, an index for the liquid phase rate of scale produced in the slab; and a second step for determining, if the index for the liquid phase rate of the scale is equal to or greater than a predetermined threshold value, that the slab has a risk of cracking into two pieces.

Description

Slab determination method, slab manufacturing method, and steel plate manufacturing method

[0001] The present invention relates to a method for determining the risk of a slab splitting into two pieces, a method for manufacturing a slab using the method, and a method for manufacturing a steel plate.

[0002] In recent years, in the field of automobiles, elements such as Si, Mn, and Ti have been added in large amounts in order to reduce the weight of car bodies and improve workability. Steels containing large amounts of elements such as Si, Mn, and Ti frequently suffer from the problem of splitting into two pieces during rolling, which significantly reduces productivity. Here, splitting into two pieces refers to a phenomenon in which a crack occurs in the center of the thickness of a steel sheet, causing the hot-rolled steel sheet to split into two pieces.

[0003] Even if the cracking does not become apparent at the stage of hot-rolled steel sheet, if the hot-rolled steel sheet is shipped with an internal crack, the cracking may occur during pressing or bending. For these reasons, it is necessary to eliminate defects that cause the cracking.

[0004] The cause of double cracks is thought to be oxidation of cross-sectional defects in the slab (cross-sectional cracks present on the cut surface or voids in the final solidification area) in the heating furnace. However, not all slabs with cross-sectional defects will develop double cracks, and the incidence of double cracks varies greatly depending on the steel type.

[0005] As a technique for suppressing the splitting, Patent Document 1 discloses a continuous casting method for steel in which the slab is forcibly pressed down at the end of solidification to compress voids in the final solidification portion. Patent Document 2 discloses a method for reducing the secondary cooling flow rate so as to suppress the confinement of the residual liquid phase even in unsteady portions.

[0006] JP 2003-94154 A JP 2003-334651 A

[0007] The conventional technology could not explain the difference in occurrence of two-ply cracks depending on the steel grade, and had the following problems. The method disclosed in Patent Document 1 can eliminate cross-sectional defects in slabs. However, the reaction force generated when forcibly rolling down the slab increases, significantly reducing the lifespan of the rolls and segments. Therefore, applying this method to all steel grades increases the cost of slab production, which creates an economic problem.

[0008] In order to affect the solidified shell thickness using the method disclosed in Patent Document 2, it is necessary to reduce the amount of cooling water from an early stage of solidification, which increases inter-roll bulging due to weakened cooling and poses the risk of generating other defects such as internal cracks. For this reason, if this method is applied to all steel types, there is a risk of increasing unnecessary risks.

[0009] The present invention has been made in consideration of the above circumstances, and its purpose is to provide a slab assessment method that can determine the risk of a slab produced using molten steel splitting into two pieces based on the component concentrations of the molten steel produced in a continuous casting device.

[0010] Means for solving the above problems are as follows. [1] A method for determining a slab containing Ti, C, Si, and Mn as components and produced using a continuous casting machine, the method comprising: a first step of determining an index of a liquid phase ratio of scale formed in the slab using component concentrations of molten steel poured into a mold of the continuous casting machine; and a second step of determining the slab as a slab at risk of splitting into two pieces if the index of the liquid phase ratio of scale is equal to or greater than a predetermined threshold. [2] The method for determining a slab according to [1], wherein the index of the liquid phase ratio of scale is an index determined by the Ti concentration, C concentration, Si concentration, and Mn concentration of the molten steel. [3] The method for determining a slab according to [1] or [2], wherein the index of the liquid phase ratio of 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} (1) In the above formula (1), f is an index (-) of the liquid phase rate of the scale, [Ti] is the Ti concentration (mass%) of the molten steel, [C] is the C concentration (mass%) of the molten steel, [Si] is the Si concentration (mass%) of the molten steel, [Mn] is the Mn concentration of the molten steel, and A Ｔｉ１ , A Ｃ , A Ｓｉ , A Ｍｎ , AＴｉ２ and A Ｓｉ／Ｍｎis a parameter. [4] The method for determining a slab according to any one of [1] to [3], wherein, in the second step, the slab is determined to be one that does not have a risk of breaking into two pieces if the index of the liquid phase ratio of the scale is less than a predetermined threshold. [5] A method for producing a slab using a continuous casting machine, wherein, when the slab is determined to be one that has a risk of breaking into two pieces in the method for determining a slab according to any one of [1] to [3], the roll gap of reduction rolls of the continuous casting machine is changed to softly reduce the slab so that the solid phase ratio at the width center and thickness center of the slab in the casting direction is in a range of more than 0.0 and not more than 1.0. [6] A method for producing a slab using a continuous casting machine, wherein, when the slab is determined to be one that has a risk of breaking into two pieces in the method for determining a slab according to any one of [1] to [3], the temperature difference in the width direction of the slab is measured in the range from a position where the slab support rolls are no longer installed to a position where the slab is cut to determine the presence or absence of a cross-sectional defect. [7] A method for producing a slab using a continuous casting machine, wherein, when the slab is determined to have a risk of splitting into two pieces in the slab determination method according to any one of [1] to [3], the cut surface of the slab is imaged to generate image data, and the presence or absence of a cross-sectional defect is determined using the image data. [8] A method for producing a slab according to [7], wherein the cut surface is imaged using an imaging device having an IR cut filter. [9] A method for producing a slab according to any one of [6] to [8], wherein a gap in the cross-sectional defect of a slab determined to have a cross-sectional defect is welded.

[10] A method for producing a steel plate by heating and hot-rolling a slab, wherein a slab determined to have no risk of splitting into two pieces in the slab determination method according to [4] is heated and hot-rolled.

[11] A method for producing a steel plate by heating and hot-rolling a slab, wherein a slab produced by the slab determination method according to [5] is heated and hot-rolled.

[12] A method for manufacturing a steel plate by heating and hot rolling a slab, the method comprising: heating and hot rolling a slab determined to be free of the cross-sectional defect by the method for manufacturing a slab according to any one of [6] to [8].

[13] A method for manufacturing a steel plate by heating and hot rolling a slab, the method comprising heating and hot rolling a slab manufactured by the method for manufacturing a slab according to [9].

[0011] According to the present invention, by using an index of the liquid phase ratio of scale obtained from the component concentrations of molten steel, it is possible to determine the risk of splitting into two pieces in a slab produced using the molten steel. By determining the risk of splitting into two pieces in this way, it becomes possible to select and deal with slabs determined to have a risk of splitting into two pieces, thereby suppressing increases in slab production costs and the risk of increasing unnecessary risks.

[0012] Fig. 1 is a schematic side view of a continuous casting machine capable of carrying out the slab evaluation method and slab manufacturing method according to the present embodiment. Fig. 2 is a graph showing the relationship between the index f of the liquid phase ratio of scale and the incidence rate of double cracks. Fig. 3 is a graph showing the relationship between the temperature difference in the width direction and cross-sectional defects in the slab.

[0013] The present invention will be specifically described below through 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.

[0014] 1 is a schematic side view of a continuous casting apparatus 100 in which the slab evaluation method and slab manufacturing method according to this embodiment can be implemented. The continuous casting apparatus 100 has 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 installed at the bottom of the tundish 12, and a submerged nozzle 16 is installed below the sliding nozzle 14.

[0015] Molten steel 18 is poured into the mold 10 through the submerged nozzle 16. The molten steel 18 poured into the mold 10 is solidified as heat is removed from the inner surface of the mold 10, forming a solidified shell 20. This results in the formation of a cast 24 having the solidified shell 20 as its outer shell and an unsolidified layer 22 made of the molten steel 18 inside.

[0016] Below the mold 10, multiple pairs of strand support rolls 32, including support rolls 26, guide rolls 28, and pinch rolls 30, are provided. Of these, the pinch rolls 30 support the strand 24 and also serve as drive rolls for withdrawing the strand 24. Spray nozzles (not shown), such as water spray nozzles or air mist spray nozzles, are provided in the gaps between adjacent strand support rolls in the casting direction, thereby forming a secondary cooling zone. The strand 24 is cooled as it is withdrawn by cooling water sprayed from the spray nozzles in the secondary cooling zone (hereinafter also referred to as "secondary cooling water"), reducing the internal unsolidified layer 22 and growing the solidified shell 20. The strand 24 is then appropriately cooled, which advances the solidification of the unsolidified layer 22 until the strand 24 is completely solidified. A plurality of transport rolls 34 for transporting the cast strand 24 are provided downstream of the strand support rolls 32, and a strand cutter 36 for cutting the cast strand 24 into slabs 38 of a predetermined length is provided above the transport rolls 34.

[0017] A soft reduction zone 42 is provided upstream of the solidification completion position 40 of the slab 24 in the casting direction, allowing for adjustment of the roll spacing between the opposing guide rolls 28. The guide rolls constituting the soft reduction zone 42 are configured so that the roll spacing can be adjusted, and by adjusting the roll spacing, the slab 24 can be soft reduced or not soft reduced. In this embodiment, the soft reduction zone 42 is installed so that the slab 24 falls within the installation range of the soft reduction zone 42 until the solid fraction at the thickness center and width center of the slab 24 reaches at least 0.1 to 1.0. The solid fraction at the thickness center of the slab 24 refers to the solid fraction at the thickness center of the slab 24 excluding the widthwise ends of the slab, but may be represented by the solid fraction at the widthwise center and thickness center of the slab. Here, the solid fraction is an index indicating the progress of solidification and is expressed in a range from 0.0 to 1.0, with a solid fraction of 0.0 (zero) indicating unsolidified and a solid fraction of 1.0 indicating complete solidification.

[0018] The reduction gradient in the soft reduction zone 42 is expressed as the roll opening reduction amount (mm / m) per meter in the casting direction, and the reduction rate (mm / min) of the slab 24 in the soft reduction zone 42 is calculated by multiplying this reduction gradient (mm / m) by the casting speed (m / min). The soft reduction zone 42 reduces the slab 24 at a reduction rate of 0.2 mm / min to 1.0 mm / min. Spray nozzles for cooling the slab 24 are also arranged between each of the slab support rolls that make up the soft reduction zone 42. While FIG. 1 shows an example in which only guide rolls 28 are arranged in the light reduction zone 42, pinch rolls 30 may also be arranged in the light reduction zone 42. The slab support rolls 32 arranged in the light reduction zone 42 are also referred to as "reduction rolls."

[0019] A thermal camera 44 is provided in the casting direction, ranging from the position where the strand support rolls 32 no longer exist to the position where the strand cutter 36 is installed, to measure the surface temperature of the strand 24 in the width direction. The thermal camera 44 measures the temperature difference in the width direction of the strand 24. By providing the thermal camera 44 in the range from the position where the strand support rolls 32 no longer exist to the position where the strand cutter 36 is installed, the surface temperature of the strand 24 can be measured without being affected by water vapor or steam. This makes it easy to measure the temperature difference in the width direction of the strand 24. Furthermore, an illumination device 46 is provided to illuminate the cut surface of the slab 38 cut by the strand cutter 36, and an imaging device 48 is provided to capture an image of the cut surface and generate image data.

[0020] In such a continuous casting apparatus 100, if the components of the slab 38 produced without soft reduction in the soft reduction zone 42 include Ti, C, Si, and Mn, a split into two may occur in the hot-rolled steel sheet produced from the slab 38. Splitting into two is a phenomenon in which a crack occurs in the center of the thickness during rolling or at the customer's site, causing the steel sheet to split into two pieces.

[0021] The present inventors have thoroughly investigated the cause of the occurrence of lamination in a hot-rolled steel sheet produced by hot-rolling a slab 38 containing Ti, C, Si, and Mn as its components. As a result, they have found that the occurrence of lamination is closely related to the presence of a cross-sectional defect in the slab 38 and a high liquid phase fraction of scale formed in the high-temperature environment of a heating furnace. Therefore, even if a cross-sectional defect is present in the slab 38, the hot-rolled steel sheet will not be subjected to lamination if the liquid phase fraction of the scale formed in the high-temperature environment of a heating furnace is low.

[0022] An index of the liquid phase ratio of scale generated in a high-temperature environment in a heating furnace can be determined using the Ti concentration, C concentration, Si concentration, and Mn concentration, which are the component concentrations of the molten steel 18. Therefore, in the slab determination method according to this embodiment, a first step is performed in which an index of the liquid phase ratio of scale is determined using the Ti concentration, C concentration, Si concentration, and Mn concentration, which are the component concentrations of the molten steel 18 poured into the mold 10. If the index of the liquid phase ratio of scale determined in the first step is equal to or greater than a predetermined threshold, a second step is performed in which the slab 38 is determined to be at risk of splitting into two slabs. Specifically, in the first step, the index f of the liquid phase ratio of scale is determined using the Ti concentration, C concentration, Si concentration, and Mn concentration of the molten steel 18 and the following equation (1):

[0023] f = A Ｔｉ１ × [Ti] × {A Ｃ ×[C]+A Ｓｉ × [Si] + A Ｍｎ ×[Mn] + A Ｔｉ２ × [Ti] + A Ｓｉ／Ｍｎ ×([Si] / [Mn]) ０．５} (1) In the above formula (1), f is an index (-) of the liquid phase ratio of 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 , A Ｓｉ／Ｍｎ is a parameter. (-) indicates that it is dimensionless. For example, when the temperature inside the heating furnace is 1200°C, the parameter in the above formula (1) is Ｔｉ１ is 8.2, and A Ｃ is -0.7, and A Ｓｉ is 2.0, and A Ｍｎ is -0.5, and A Ｔｉ２ is 23, and A Ｓｉ／Ｍｎ is 2.7.

[0024] In the second step, a slab having a liquid phase ratio index f of scale calculated using the formula (1) equal to or greater than a predetermined threshold is determined to be at risk of splitting into two layers. On the other hand, a slab having a liquid phase ratio index f of scale less than a predetermined threshold may be determined to be at no risk of splitting into two layers.

[0025] 2 is a graph showing the relationship between the index f of the liquid phase ratio of scale and the incidence rate of double cracks. The horizontal axis of FIG. 2 is the index f(-) of the liquid phase ratio of scale, which is a value calculated using the above formula (1) and the Ti concentration, C concentration, Si concentration, and Mn concentration of the molten steel 18. The parameters of formula (1) are A Ｔｉ１ : 8.2, A Ｃ : -0.7, A Ｓｉ : 2.0, A Ｍｎ : -0.5, A Ｔｉ２ : 23, A Ｓｉ／Ｍｎ :2.7 was used.

[0026] The vertical axis of Figure 2 represents the incidence rate (%) of double-lamination cracks. The incidence rate of double-lamination cracks is the incidence rate (%) of double-lamination cracks that occurred in a hot-rolled steel sheet produced by hot-rolling a slab that was cast without soft reduction using molten steel 18 having the component concentrations for which the scale liquid phase fraction index f was calculated. As shown in Figure 2, for slabs with a scale liquid phase fraction index f of 1.00 or more, the incidence rate of double-lamination cracks in the hot-rolled steel sheet produced by hot-rolling the slab was 0.2% or more. On the other hand, for slabs with a scale liquid phase fraction index f of less than 1.00, the incidence rate of double-lamination cracks in the hot-rolled steel sheet produced by hot-rolling the slab was 0%. From these results, the scale liquid phase fraction index f is calculated using the component concentrations of molten steel 18, that is, Ti concentration, C concentration, Si concentration, and Mn concentration, and the above formula (1), and it is determined whether the double-lamination crack risk index f is 1.00 or more. This makes it possible to determine the risk of the slab produced from the molten steel splitting into two pieces.

[0027] If the risk of double cracking can be determined in this way, it is possible to select and respond to slabs determined to have a risk of double cracking, thereby preventing an increase in the manufacturing cost of slabs 38 and preventing unnecessary increases in risk by dealing with slabs 38 that do not have a risk of double cracking. 1.00 used to determine the risk of double cracking is an example of a threshold value for the index f of the scale liquid phase ratio. The threshold value for the index f of the scale liquid phase ratio can be determined in advance by investigating the relationship between the index f of the scale liquid phase ratio and the occurrence rate of double cracking, as shown in Figure 2.

[0028] When a slab is determined to have a risk of splitting into two pieces in the slab determination method according to this embodiment, if a cross-sectional defect exists in the slab, there is a high possibility that a split will occur in the hot-rolled steel sheet produced by hot rolling. Therefore, in the slab manufacturing method according to this embodiment, for a slab determined to have a risk of splitting into two pieces in the slab determination method, the continuous casting conditions are changed so that a cross-sectional defect will not occur in the slab.

[0029] For example, it is preferable to suppress cross-sectional defects in the slab produced by narrowing the roll gap between the reduction rolls in the soft reduction zone 42 of the continuous casting apparatus 100 to a value smaller than the thickness of the slab 24 and applying a soft reduction rate of 0.2 mm / min to 1.0 mm / min. This soft reduction is preferably applied so that the solid fraction at the center of the width and thickness of the slab 24 in the casting direction is greater than 0.0 and less than 1.0. The soft reduction may also be applied so that the solid fraction is greater than 0.0 and less than 0.7 (flow limit solid fraction). This prevents the occurrence of cross-sectional defects in the slab 38.

[0030] In this way, by changing the continuous casting conditions to produce a slab 38 without cross-sectional defects, even if scale with a high liquid phase ratio occurs in the high-temperature environment inside the heating furnace, it is possible to prevent the hot-rolled steel sheet produced by hot rolling the slab from splitting into two pieces.

[0031] On the other hand, if the slab is determined to have no risk of splitting into two pieces using the slab determination method according to this embodiment, even if the slab has a cross-sectional defect, the splitting will not occur in the hot-rolled steel sheet produced by hot rolling the slab. For this reason, for slabs determined to have no risk of splitting into two pieces using the above slab determination method, it is preferable to change the continuous casting conditions so that the roll spacing of the guide rolls 28 in the light reduction zone 42 is widened to the thickness of the slab 24 so as not to apply a soft reduction to the slab 24. This suppresses a decrease in the lifespan of the guide rolls and roll segments in the light reduction zone 42, thereby suppressing an increase in the manufacturing cost of the slab 38.

[0032] As described above, even if a hot-rolled steel sheet is manufactured using a slab manufactured by the slab manufacturing method according to this embodiment, the steel sheet does not undergo double cracking. Therefore, by heating the slab in a heating furnace and hot-rolling it in a rolling mill, a hot-rolled steel sheet that does not undergo double cracking during rolling or at the customer's site can be manufactured.

[0033] Next, we will explain the above formula (1) used to calculate the index f of the liquid phase ratio of scale. Slabs with a high liquid phase ratio of scale in the high-temperature environment of a heating furnace have a high frequency of occurrence of splitting in the hot-rolled steel sheet produced by hot rolling the slab. Liquid phase scale has a faster oxygen diffusion rate than solid phase scale, so it diffuses sufficiently within the cross-sectional defects and oxidizes. When this liquid phase scale is stretched during hot rolling, splitting occurs during rolling of the hot-rolled steel sheet or at the customer's site. Furthermore, according to the inventors' research, the liquid phase ratio of scale is fayalite (Fe ２ SiO ４ It was confirmed that the liquid phase ratio of the scale is particularly high when Ti is added.

[0034] Equation (1) for determining the index f of the liquid phase ratio of scale is a regression equation created in consideration of the degree to which the component concentrations of molten steel 18 affect the liquid phase ratio of the scale. The regression equation was created by first calculating the liquid phase ratio of scale in a predetermined high-temperature environment in a heating furnace using thermodynamic calculations, and then determining the regression equation and each parameter as a base by considering the component dependency of this liquid phase ratio. FeO—SiO ２ -MnO-TiO２ On the system scale, fayalite (Fe ２ SiO ４ ) the melting point becomes lower. Therefore, the higher the Si concentration in the molten steel 18, the more likely it is that low-melting-point scale will occur. On the other hand, the higher the amount of C and Mn, which are the added elements that are oxidized, the more likely it is that SiO ２ Since this makes it difficult for Mn to form, the parameters for C concentration and Mn concentration in the regression equation were set to negative values. The amount of Mn added is large, and depending on the ratio with Si, the scale that forms may switch to being mainly MnO. To take this into consideration, the term Si concentration / Mn concentration was introduced into the regression equation. Furthermore, Ti has the effect of lowering the melting point of fayalite. For this reason, the Ti concentration was also introduced into the regression equation as a multiplier for all terms.

[0035] After determining the regression equation and each parameter based on these ideas, the equation was compared with the actual occurrence rate of double-ply cracks in steel sheets produced by heating to 1200°C in a heating furnace and hot rolling. Then, the boundary between the index f of the liquid phase ratio of scale where double-ply cracks do not occur and the index f of the liquid phase ratio of scale where double-ply cracks do occur was normalized to 1.00. An example of the regression equation determined in this way is the above-mentioned equation (1), and examples of the parameters at that time are A Ｔｉ１ : 8.2, A Ｃ : -0.7, A Ｓｉ : 2.0, A Ｍｎ : -0.5, A Ｔｉ２ : 23, A Ｓｉ／Ｍｎ :2.7.

[0036] In the above example, the regression equation for determining the index f of the liquid phase fraction of scale and the respective parameters are determined by calculating the liquid phase fraction of scale by thermodynamic calculation, but the method for determining the regression equation for determining the index f of the liquid phase fraction of scale and the respective parameters is not limited to this. For example, the regression equation and the respective parameters may be determined by actually generating scale in a laboratory experiment and investigating the state of the scale. Furthermore, the regression equation and the respective parameters may be determined from the relationship between the occurrence rate of double-layer crack defects in an actual machine and the component concentrations of molten steel 18.

[0037] In the slab evaluation method according to the present embodiment, an example has been shown in which the component concentrations of molten steel 18 poured into the mold 10 of the continuous casting apparatus 100 are used to evaluate the risk of a slab to be produced by the continuous casting apparatus 100 splitting into two pieces, but the present invention is not limited to this. The slab evaluation method according to the present embodiment can evaluate the risk of a slab to be split into two pieces not only for a slab to be produced by the continuous casting apparatus 100, but also for a slab produced in the past. In this case, the component concentrations of the slab produced in the past can be used instead of the component concentrations of the molten steel 18 poured into the mold 10 to determine an index of the liquid phase ratio of scale that will form on the slab in the high-temperature environment of a heating furnace.

[0038] The present invention is not limited to the above embodiment and various modifications can be made. In the slab manufacturing method according to the present embodiment, an example has been shown in which the continuous casting conditions are changed to apply a soft reduction to a slab 38 determined to have a risk of splitting into two pieces in the second process, but the present invention is not limited to this. For a slab 38 determined to have a risk of splitting into two pieces in the second process, the presence or absence of a cross-sectional defect in the slab 38 may be determined, and the cross-sectional defect in the slab 38 determined to have a 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 cross-sectional defect determination methods.

[0039] <Method 1 for Determining Cross-Sectional Defects> First, method 1 for determining cross-sectional defects in a slab will be described. As a result of extensive research, the inventors have found that as the temperature difference in the width direction of the slab 24 at the final stage of solidification increases, cross-sectional defects are more likely to occur in the slab 38 produced by cutting the slab 24. The temperature difference in the width direction of the slab 24 is measured in the range from the position where the slab support rolls 32 are no longer provided in the casting direction to the position where the slab cutter 36 is provided.

[0040] Figure 3 is a graph showing the relationship between the temperature difference in the width direction and cross-sectional defects of the slab. In Figure 3, the horizontal axis represents the width dimension (mm) of the slab, 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 slab 24 in the width direction and the maximum temperature over the entire width direction of the slab (excluding the range of ±100 mm from the center in the width direction). In the graph, ◯ indicates a production example of a slab 38 in which no cross-sectional defects were found, and × indicates a production example of a slab 38 in which cross-sectional defects were found.

[0041] 3, when the temperature difference in the width direction of the slab 24 is 80° C. or more, cross-sectional defects are likely to occur in the slab 38, and when the temperature difference in the width direction of the slab 24 is 100° C. or more, cross-sectional defects occur in the slab 38. From this result, 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 slab 24 in the range from when solidification of the slab 24 is complete to the position where the slab cutter 36 is installed.

[0042] Specifically, if the second step determines that the slab is at risk of splitting into two pieces, the temperature difference in the width direction of the slab 24 is measured using a thermal camera 44, and if the temperature difference exceeds 80°C, it is determined that a cross-sectional defect has occurred in the slab 38 to be produced. On the other hand, if the temperature difference is 80°C or less, it is determined that no cross-sectional defect has occurred in the slab 38 to be produced.

[0043] The temperature difference of 80°C is an example of a predetermined threshold value for 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 cross-sectional defects, as shown in the graph in Figure 3. In this way, in the slab manufacturing method according to this embodiment, the presence or absence of cross-sectional defects may be determined for a slab that is determined to have a risk of splitting into two pieces in the second step using the cross-sectional defect determination method 1.

[0044] <Method 2 for Determining Cross-Sectional Defects> Next, method 2 for determining cross-sectional defects in a slab will be described. In method 2 for determining cross-sectional defects, the presence or absence of cross-sectional defects in the slab 38 is determined using image data generated by an imaging device capturing an image of the cut surface cut by the slab cutting machine 36. When a cross-sectional defect occurs in the slab 38, a crack occurs in the cut surface of the slab 38. Therefore, the presence or absence of cross-sectional defects in the slab 38 can be determined by detecting the crack in the cut surface using image data generated by capturing an image of the cut surface of the slab 38.

[0045] The detection of cracks may be performed by an operator visually checking the image data, or by image processing of the image data. Since cracks have a lower brightness than other parts, for example, the image data is binarized using a brightness threshold that can distinguish the crack from other parts to determine the width and height dimensions of the crack, and these dimensions can be compared with the past actual dimensions of the crack to detect the presence or absence of a crack.

[0046] It is preferable that the imaging device 48 have an IR cut filter. Because the cut surface of the slab 38 is red-hot, if the cut surface is directly imaged, the difference in pixel brightness between the cracked area and the other areas will be small, making it difficult to detect the crack. In contrast, by using an imaging device equipped with an IR cut filter that cuts out light with wavelengths of 700 nm or more, the influence of red-hot light can be reduced, enabling reliable detection of cracks on the cut surface.

[0047] In addition, it is preferable to illuminate the cut surface of the slab 38 with the lighting device 46. Furthermore, it is preferable that the lighting device 46 illuminate the cut surface of the slab 38 at an angle from either above or below with respect to the thickness direction of the slab 38. In this way, by illuminating the cut surface from the lighting device 46 at an angle, a shadow is cast on the crack, making it possible to detect the crack more reliably.

[0048] If a cross-sectional defect is present in a slab determined to have a risk of splitting into two pieces in the second process, there is a high possibility that splitting into two pieces will occur in the hot-rolled steel sheet produced by hot rolling. Therefore, if a cross-sectional defect is determined to exist by cross-sectional defect determination method 1 or 2, the crack that has occurred on the cut surface of the slab 38 is welded to close the gap. This prevents scale with a high liquid phase ratio from entering the gap in the cross-sectional defect, so that even if the slab is determined to have a risk of splitting into two pieces and a cross-sectional defect, it becomes possible to prevent splitting into two pieces in the hot-rolled steel sheet produced by hot rolling.

[0049] On the other hand, if it is determined that there is no cross-sectional defect by cross-sectional defect determination method 1 or 2, a split into two will not occur in the hot-rolled steel sheet manufactured using the slab 38. Therefore, the slab 38 determined to have no cross-sectional defect by cross-sectional defect determination method 1 or 2 may be used as is for manufacturing hot-rolled steel sheet. By making a determination by cross-sectional defect determination method 1 or 2 in this way, it is possible to reduce the number of slabs removed from the manufacturing line for welding the cut surfaces of the slab 38, thereby suppressing an increase in the manufacturing cost of the slab.

[0050] Examples in which the effects of the present invention were confirmed are described below. Slabs were produced using the continuous casting apparatus 100 for steel types A to C. The component compositions, the index f of the scale liquid phase ratio, whether soft reduction was performed, and the incidence of double cracking for steel types A to C are shown in Table 1 below. The index f of the scale liquid phase ratio is a value calculated using the following formula (2).

[0051] f=8.2×[Ti]×{-0.7×[C]+2.0×[Si]-0.5×[Mn]+23×[Ti]+2.7×([Si] / [Mn]) ０．５} ... (2)

[0052]

[0053] In Table 1, multiple tests are conducted on the same steel grade in order to evaluate variations in chemical composition. In Table 1, the double-crack occurrence rate indicates the percentage of double-cracks occurring in the hot-rolled steel sheets manufactured using each slab.

[0054] Test Nos. 1 to 3 and 5 to 7 demonstrate the validity of the determination based on the scale liquid fraction index f due to component variations. As shown in Table 1, when a slab with a scale liquid fraction index f of 1.00 or more was used, the slab was heated to 1200°C in a heating furnace and hot-rolled to produce a hot-rolled steel sheet, and a double crack occurred. On the other hand, when a slab with a scale liquid fraction index f of less than 1.00 was used, the slab was heated to 1200°C in a heating furnace and hot-rolled to produce a hot-rolled steel sheet, and a double crack did not occur. From these results, the component concentrations of the molten steel before being cast in a continuous casting machine were used to calculate the scale liquid fraction index f of the slab produced from the molten steel. Then, by determining whether the calculated index f is equal to or greater than a predetermined threshold, it was confirmed that the risk of double cracking in a hot-rolled steel sheet produced by hot-rolling the slab can be determined.

[0055] Test Nos. 4, 8, and 9 show the results of soft reduction performed under changed casting conditions on slabs that had a scale liquid phase ratio index f of 1.00 or more and were evaluated as having a risk of splitting into two pieces. Although test Nos. 4, 8, and 9 were evaluated as having a risk of splitting into two pieces, soft reduction was performed to eliminate cross-sectional defects in the slabs. The soft reduction rates were No. 4: 0.24 mm / min, No. 8: 0.66 mm / min, and No. 9: 0.96 mm / min. Thus, for the slabs Nos. 4, 8, and 9 that had been soft reduced, the slabs were heated to 1200°C in a heating furnace and hot-rolled to produce hot-rolled steel sheets, and no splitting occurred in the hot-rolled steel sheets. These results confirmed that by applying appropriate light reduction to slabs determined to be at risk of splitting into two pieces at the end of solidification, it is possible to prevent splitting into two pieces in the hot-rolled steel plate produced by hot rolling the slab.

[0056] REFERENCE SIGNS LIST 10 Mold 12 Tundish 14 Sliding nozzle 16 Submerged entry nozzle 18 Molten steel 20 Solidified shell 22 Unsolidified layer 24 Strand 26 Support roll 28 Guide roll 30 Pinch roll 32 Strand support roll 34 Transport roll 36 Strand cutter 38 Slab 40 Solidification completion position 42 Soft reduction zone 44 Thermocamera 46 Lighting device 48 Imaging device 100 Continuous casting device

Claims

1. A method for judging a slab containing Ti, C, Si and Mn as components and produced using a continuous casting machine, comprising: a first step of determining an index of the liquid phase rate of scale formed on the slab using the component concentrations of molten steel poured into a mold of the continuous casting machine; and a second step of judging the slab to be at risk of splitting into two pieces if the index of the liquid phase rate of the scale is equal to or greater than a predetermined threshold value.

2. A method for evaluating a slab as described in claim 1, wherein the index of the liquid phase rate of the scale is an index determined by the Ti concentration, C concentration, Si concentration and Mn concentration of the molten steel.

3. The method for judging a slab according to claim 1 or 2, wherein the index of the liquid phase ratio 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 In the above formula (1), f is an index (-) of the liquid phase rate of the scale, [Ti] is a Ti concentration (mass%) of the molten steel, [C] is a C concentration (mass%) of the molten steel, [Si] is a Si concentration (mass%) of the molten steel, [Mn] is a Mn concentration (mass%) of the molten steel, and A Ｔｉ１ , A Ｃ , A Ｓｉ , A Ｍｎ , A Ｔｉ２ and A Ｓｉ／Ｍｎ is a parameter.

4. A method for determining a slab as described in any one of claims 1 to 3, wherein in the second step, if the index of the liquid phase ratio of the scale is less than a predetermined threshold value, the slab is determined to be a slab with no risk of cracking into two pieces.

5. A method for producing a slab using a continuous casting device, wherein, when a slab is determined to be at risk of splitting into two pieces using the slab determination method described in any one of claims 1 to 3, the roll opening of the reduction rolls of the continuous casting device is changed so that the solid phase ratio at the center of the width and center of the thickness of the slab in the casting direction is in the range of greater than 0.0 and not exceeding 1.0, to lightly reduce the slab.

6. A method for manufacturing a slab using a continuous casting device, comprising the steps of: determining, in the method for determining a slab as set forth in any one of claims 1 to 3, that if a slab is determined to be at risk of splitting into two pieces, measuring the temperature difference in the width direction of the slab in the range from the position where the slab support rolls are no longer installed to the position where the slab is cut, to determine whether or not there is a cross-sectional defect.

7. A method for manufacturing a slab using a continuous casting device, comprising the steps of: if a slab is determined to be at risk of breaking into two pieces in the slab judgment method described in any one of claims 1 to 3, imaging the cut surface of the slab to generate image data, and using the image data to judge whether or not there is a cross-sectional defect.

8. The method of manufacturing a slab according to claim 7, wherein the cut surface is imaged using an imaging device having an IR cut filter.

9. A method for manufacturing a slab according to any one of claims 6 to 8, further comprising welding a gap in a cross-sectional defect in a slab determined to have a cross-sectional defect.

10. A method for manufacturing steel plate, comprising heating a slab and hot rolling it to manufacture the steel plate, comprising heating and hot rolling a slab that has been determined to have no risk of splitting into two pieces by the slab determination method described in claim 4.

11. A method for producing a steel plate, comprising heating a slab and hot rolling it to produce a steel plate, the method comprising heating and hot rolling the slab produced by the method for producing a slab according to claim 5.

12. A method for manufacturing steel plate, comprising heating a slab and hot rolling it to manufacture a steel plate, the method comprising heating and hot rolling a slab 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.

13. A method for producing a steel plate, comprising heating a slab and hot rolling it to produce a steel plate, the method comprising heating and hot rolling a slab produced by the method for producing a slab according to claim 9.

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