Methods for determining steel billets, methods for manufacturing steel billets, and methods for manufacturing steel plates.

Abstract

This invention provides a method for determining the risk of delamination in steel billets by analyzing the composition concentration of molten steel produced in a continuous casting apparatus. The method for determining the risk of delamination in steel billets produced using this molten steel is specifically for steel billets containing Ti, C, Si, and Mn as components and manufactured using a continuous casting apparatus. The method includes a first step and a second step. The first step involves determining the liquid phase ratio of the oxide film formed on the steel billet by analyzing the composition concentration of the molten steel injected into the mold of the continuous casting apparatus. The second step involves determining that if the liquid phase ratio of the oxide film is above a predetermined threshold, the steel billet is considered to have a risk of delamination.

Description

Method for Determining Slab, Method for Manufacturing Slab, and Method for Manufacturing Steel Plate The present invention relates to a method for determining a slab for determining the risk of layer cracks, a method for manufacturing a slab using the determination method, and a method for manufacturing a steel plate. In recent years, in the automotive field, from the viewpoints of vehicle body weight reduction and workability, a large amount of components such as Si, Mn, and Ti have been added. When rolling steel containing a large amount of components such as Si, Mn, and Ti, layer crack problems often occur, significantly reducing productivity. Here, the so-called "layer crack" refers to a phenomenon in which cracks appear in the central part of the thickness of a steel plate, and the hot-rolled steel plate breaks into two layers. Even if layer cracks do not appear at the hot-rolled steel plate stage, if the hot-rolled steel plate is shipped in a state where cracks have occurred inside, layer cracks may still occur during stamping or bending processing. According to these situations, it is necessary to eliminate the defects that cause layer cracks. The cause of layer cracks is considered to be due to the oxidation of cross-sectional defects (cross-sectional cracks existing in the cut surface or voids in the final solidification part) of the slab in the heating furnace. However, not all slabs with cross-sectional defects will produce layer cracks, and the occurrence rate of layer cracks varies greatly depending on the steel type. As a technique for suppressing layer cracks, Patent Document 1 discloses a continuous casting method for steel in which the slab is forcibly pressed at the end of solidification to compress the voids in the final solidification part. Patent Document 2 discloses a method of reducing the secondary cooling flow rate in such a way that the entrapment of the remaining liquid phase can be suppressed even in unstable parts. [Prior Art Documents] [Patent Documents] Patent Document 1: Japanese Patent Laid-Open No. 2003-94154 Patent Document 2: Japanese Patent Laid-Open No. 2003-334651 (Problems to be Solved by the Invention) In the prior art, it is unable to explain the difference in the occurrence of layer cracks caused by steel types, and there are the following problems. By using the method disclosed in Patent Document 1, the cross-sectional defects of the slab can be eliminated. However, the reaction force when forcibly pressing the slab becomes large, significantly reducing the life of the roll or segment, so if it is applied to all steel types, economic problems such as an increase in the manufacturing cost of the slab will occur. When using the method disclosed in Patent Document 2 to affect the thickness of the solidified shell, it is necessary to reduce the cooling water volume from an earlier stage of solidification, and there is a risk of an increase in bulging between rolls caused by weak cooling, resulting in other defects such as internal fractures. Therefore, when applied to all steel types, there is a danger of increasing unnecessary risks. The present invention has been completed in view of this situation, and its purpose is to provide a method for determining a steel billet, which can determine the risk of laminar cracking of the steel billet manufactured using the molten steel based on the component concentration of the molten steel manufactured by the continuous casting device. (Technical means for solving the problem) The means for solving the above problems are as follows. [1] A method for determining a steel billet, which is a method for determining a steel billet containing Ti, C, Si, and Mn as components and manufactured using a continuous casting device; it includes a first step and a second step. The first step is to use the component concentration of the molten steel injected into the mold of the continuous casting device to obtain the liquid phase rate index of the oxide film (scale) generated on the above steel billet; the second step is to determine the above steel billet as a steel billet with a risk of laminar cracking when the liquid phase rate index of the above oxide film is equal to or higher than a predetermined threshold value. [2] The method for determining a steel billet according to [1], wherein the liquid phase rate index of the above oxide film is an index determined by the Ti concentration, C concentration, Si concentration, and Mn concentration of the above molten steel. [3] The method for determining a steel billet according to [1] or [2], wherein the liquid phase rate index of the above oxide film 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 the liquid phase rate index (-) of the above oxide film, [Ti] is the Ti concentration (mass%) of the above molten steel, [C] is the C concentration (mass%) of the above molten steel, [Si] is the Si concentration (mass%) of the above molten steel, [Mn] is the Mn concentration (mass%) of the above molten steel, A Ti1 、A C 、A Si 、A Mn 、A Ti2 及A Si / Mnis a parameter. [4] The method for determining a steel billet as described in any one of [1] to [3], wherein in the above-mentioned second step, when the liquid phase rate index of the above-mentioned oxide film does not reach a predetermined threshold value, it is determined that the above-mentioned steel billet is a steel billet without the risk of laminar cracking. [5] A method for manufacturing a steel billet, which is a method for manufacturing a steel billet using a continuous casting device. When it is determined in the method for determining a steel billet as described in any one of [1] to [3] that the steel billet has a risk of laminar cracking, when the solid phase rate at the center of the width and the center of the thickness of the ingot in the casting direction is greater than 0.0 and within the range of 1.0 or less, the roll opening of the reduction roll of the above-mentioned continuous casting device is changed, and the above-mentioned ingot is lightly pressed. [6] A method for manufacturing a steel billet, which is a method for manufacturing a steel billet using a continuous casting device. When it is determined in the method for determining a steel billet as described in any one of [1] to [3] that the steel billet has a risk of laminar cracking, within the range from the position where the ingot support roll is not provided to the position where the ingot is cut, the temperature difference in the width direction of the above-mentioned ingot is measured and it is determined whether there is a cross-sectional defect. [7] A method for manufacturing a steel billet, which is a method for manufacturing a steel billet using a continuous casting device. When it is determined in the method for determining a steel billet as described in any one of [1] to [3] that the steel billet has a risk of laminar cracking, the cut surface of the above-mentioned steel billet is photographed to generate image data, and the above-mentioned image data is used to determine whether there is a cross-sectional defect. [8] The method for manufacturing a steel billet as described in [7], which photographs the above-mentioned cut surface using a photographing device having an infrared cut filter (IR Cut Filter). [9] The method for manufacturing a steel billet as described in any one of [6] to [8], which welds the void of the cross-sectional defect to the steel billet determined to have the cross-sectional defect.

[10] A method for manufacturing a steel plate, which is a method for manufacturing a steel plate by heating and hot rolling a steel billet. It heats and hot rolls the steel billet determined to have no risk of laminar cracking in the method for determining a steel billet as described in [4].

[11] A method for manufacturing a steel plate, which is a method for manufacturing a steel plate by heating and hot rolling a steel billet. It heats and hot rolls the steel billet manufactured in the method for manufacturing a steel billet as described in [5].

[12] A method for manufacturing a steel plate, which is a method for manufacturing a steel plate by heating and hot rolling a steel billet. It heats and hot rolls the steel billet determined to have no above-mentioned cross-sectional defect in the method for manufacturing a steel billet as described in any one of [6] to [8].

[13] A method for manufacturing a steel plate, which is a method for manufacturing a steel plate by heating and hot rolling a steel billet. It heats and hot rolls the steel billet manufactured in the method for manufacturing a steel billet as described in [9]. (Effect compared with the prior art) According to the present invention, by using the liquid phase rate index of the oxide film obtained based on the component concentration of the molten steel, the lamellar tearing risk of the steel billet manufactured using the molten steel can be determined. By determining the lamellar tearing risk in this way, it is possible to select a response to the steel billet determined to have a lamellar tearing risk, and thus it is possible to suppress an increase in the manufacturing cost of the steel billet or increase the danger of an unnecessary risk. Hereinafter, the present invention will be specifically described by embodiments of the present invention. The following embodiments show a preferred example of the present invention, but the present invention is not limited by these embodiments in any way. FIG. 1 is a side schematic view of a continuous casting apparatus 100 capable of implementing the method for determining a steel billet and the method for manufacturing a steel billet according to the present embodiment. The continuous casting apparatus 100 includes a mold 10, a tundish 12 provided above the mold 10, and a plurality of pairs of casting support rolls 32 arranged below the mold 10 in a plurality. A sliding nozzle 14 for adjusting the flow rate of the molten steel 18 is provided at the bottom of the tundish 12, and a submerged nozzle 16 is provided below the sliding nozzle 14. The molten steel 18 is injected into the mold 10 through the submerged nozzle 16. The molten steel 18 injected into the mold 10 is heat-removed from the inner surface of the mold 10 and solidifies to form a solidified shell 20. Thereby, a casting 24 having the solidified shell 20 as an outer shell and an unfrozen layer 22 formed of the molten steel 18 inside is formed. A plurality of pairs of casting support rolls 32 including support rolls 26, guide rolls 28, and pinch rolls 30 are provided below the mold 10. Among them, the pinch roll 30 is also a drive roll for supporting the casting 24 and pulling the casting 24. In the gap between the casting support rolls adjacent in the casting direction, an atomizing nozzle (not shown) such as a water mist nozzle or an air mist nozzle is provided, thereby constituting a secondary cooling zone. By using the cooling water (hereinafter also referred to as "secondary cooling water") sprayed from the atomizing nozzle in the secondary cooling zone, the casting 24 is cooled while being pulled, and the unfrozen layer 22 inside is reduced, and the solidified shell 20 grows. Then, the casting 24 is appropriately cooled, and the solidification of the unfrozen layer 22 progresses, and the casting 24 is completely solidified. A plurality of conveying rolls 34 for conveying the cast casting 24 are provided on the downstream side of the casting support rolls 32, and a casting cutting machine 36 for cutting the cast casting 24 into a steel billet 38 of a predetermined length is provided above the conveying rolls 34. Above the casting direction of the solidification position 40 of the cast strip 24, a soft reduction zone 42 for adjusting the roll gap of the opposing guide rolls 28 is provided. The guide rolls constituting the soft reduction zone 42 are configured to be adjustable in roll gap. By adjusting the roll gap, a soft reduction can be applied to the cast strip 24, or no soft reduction can be applied. In this embodiment, for the cast strip 24 with a solid fraction of at least 0.1 to 1.0 at the thickness center and width center of the cast strip 24, the soft reduction zone 42 is arranged in such a way that the cast strip 24 enters the set range of the soft reduction zone 42. The so-called "solid fraction at the thickness center of the cast strip 24" refers to the solid fraction at the thickness center of the cast strip excluding the ends in the width direction of the cast strip, but it can also be represented by the solid fraction at the center in the width direction and the thickness center of the cast strip. Here, the so-called "solid fraction" is an index indicating the progress of solidification, and the solid fraction is expressed in the range of 0.0 to 1.0. A solid fraction = 0.0 (zero) means not solidified, and a solid fraction = 1.0 means completely solidified. The reduction gradient of the soft reduction zone 42 is expressed by the roll opening reduction amount (mm / m) per 1 m in the casting direction. The reduction speed (mm / min) of the cast strip 24 in the soft reduction zone 42 is obtained by multiplying the reduction gradient (mm / m) by the casting speed (m / min). The soft reduction zone 42 reduces the cast strip 24 at a reduction speed of 0.2 mm / min or more and 1.0 mm / min or less. Atomizing nozzles for cooling the cast strip 24 are also arranged between the respective cast strip support rolls constituting the soft reduction zone 42. In FIG. 1, an example of arranging only the guide rolls 28 in the soft reduction zone 42 is shown, but pinch rolls 30 can also be arranged in the soft reduction zone 42. The cast strip support roll 32 arranged in the soft reduction zone 42 is also called a "reduction roll". In the casting direction, in the range from the position where the cast strip support roll 32 is not provided to the position where the cast strip cutting machine 36 is provided, an infrared thermal imager 44 for measuring the surface temperature in the width direction of the cast strip 24 is provided. The infrared thermal imager 44 measures the temperature difference in the width direction of the cast strip 24. By arranging the infrared thermal imager 44 in the range from the position where the cast strip support roll 32 is not provided to the position where the cast strip cutting machine 36 is provided, it can measure the surface temperature of the cast strip 24 without being affected by water vapor or hot air. Thereby, it can easily measure the temperature difference in the width direction of the cast strip 24. Also, it is provided with: an illumination device 46 for illuminating the cutting surface of the steel billet 38 cut by the cast strip cutting machine 36, and a photographing device 48 for photographing the cutting surface and generating image data. In this continuous casting apparatus 100, if the composition of the steel billet 38 manufactured without applying soft reduction in the soft reduction zone 42 contains Ti, C, Si, and Mn, the hot-rolled steel sheet manufactured from the steel billet 38 will have laminar tearing. The so-called "laminar tearing" refers to the phenomenon that cracks appear at the center of the thickness during rolling or at the customer end, and the steel sheet breaks into two layers. The inventors of the present invention have conducted in-depth research on the cause of laminations in a hot-rolled steel sheet manufactured by hot-rolling a steel billet 38 containing Ti, C, Si, and Mn in its components. As a result, it has been found that the laminations are highly related to the fact that the steel billet 38 has cross-sectional defects and the liquid phase ratio of the oxide film formed in the high-temperature environment in the heating furnace is high. Therefore, it is hypothesized that even if the steel billet 38 has cross-sectional defects, as long as the liquid phase ratio of the oxide film formed in the high-temperature environment in the heating furnace is reduced, the hot-rolled steel sheet will not develop laminations. The liquid phase ratio index of the oxide film formed in the high-temperature environment in the heating furnace can be obtained using the Ti concentration, C concentration, Si concentration, and Mn concentration of the component concentrations of the molten steel 18. Therefore, in the determination method of the steel billet in the present embodiment, a first step of obtaining the liquid phase ratio index of the oxide film is implemented by using the Ti concentration, C concentration, Si concentration, and Mn concentration of the component concentrations of the molten steel 18 injected into the mold 10. When the liquid phase ratio index of the oxide film obtained in the first step is equal to or higher than a predetermined threshold value, a second step of determining that the steel billet 38 is a steel billet having a risk of laminations is implemented. Specifically, in the above first step, the Ti concentration, C concentration, Si concentration, Mn concentration of the molten steel 18, and the following formula (1) are used to obtain the liquid phase ratio index f of the oxide film. 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 the liquid phase ratio index (-) of the oxide film. [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 / Mnis a parameter. (-) indicates dimensionless. The parameters in the above formula (1) are, for example, when the temperature in the heating furnace is 1200 °C, A Ti1 is 8.2, A C is -0.7, A Si is 2.0, A Mn is -0.5, A Ti2 is 23, A Si / Mn is 2.7. In the above step 2, the steel billets with the liquid phase rate index f of the oxide film obtained by using the above formula (1) being above a predetermined threshold are determined as steel billets with a risk of centerline segregation. On the other hand, the steel billets with the liquid phase rate index f of the oxide film being less than the predetermined threshold can also be determined as steel billets without a risk of centerline segregation. Figure 2 is a chart showing the relationship between the liquid phase rate index f of the oxide film and the incidence rate of centerline segregation. The horizontal axis of Figure 2 is the liquid phase rate index f (-) of the oxide film, and it 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 using A Ti1 : 8.2, A C : -0.7, A Si : 2.0, A Mn : -0.5, A Ti2 : 23, A Si / Mn : 2.7. The vertical axis of Figure 2 is the incidence rate of centerline segregation (%). The incidence rate of centerline segregation is the incidence rate (%) of centerline segregation occurring in the hot-rolled steel sheet manufactured by hot-rolling the steel billets cast without soft reduction using the molten steel 18 with the component concentrations for which the liquid phase rate index f of the oxide film has been calculated. As shown in Figure 2, for the steel billets with the liquid phase rate index f of the oxide film being 1.00 or more, the incidence rate of centerline segregation of the hot-rolled steel sheet manufactured by hot-rolling the steel billets is 0.2% or more. On the other hand, for the steel billets with the liquid phase rate index f of the oxide film being less than 1.00, the incidence rate of centerline segregation of the hot-rolled steel sheet manufactured by hot-rolling the steel billets is 0%. Based on this result, the Ti concentration, C concentration, Si concentration, Mn concentration of the component concentrations of the molten steel 18 and the above formula (1) are used to obtain the liquid phase rate index f of the oxide film and determine whether the centerline segregation risk index f is 1.00 or more. Thereby, the centerline segregation risk of the steel billets manufactured from the molten steel can be determined. In this way, if the laminating risk can be determined, a method corresponding to the steel billet determined to have the laminating risk can be selected, so that an increase in the manufacturing cost of the steel billet 38 can be suppressed, or a situation where unnecessary risks are increased due to the need to also correspond to the steel billet 38 without the laminating risk can be avoided. The 1.00 used in the determination of the laminating risk is an example of the threshold value of the liquid phase ratio index f of the oxide film. The threshold value of the liquid phase ratio index f of the oxide film can be predetermined by investigating the relationship between the liquid phase ratio index f of the oxide film shown in FIG. 2 and the occurrence rate of lamination. In the determination method of the steel billet of this embodiment, when the steel billet is determined to have the laminating risk, if the steel billet has a cross-sectional defect, the possibility of lamination occurring in the hot-rolled steel sheet manufactured by hot rolling will increase. Therefore, in the manufacturing method of the steel billet of this embodiment, for the steel billet determined to have the laminating risk in the above-mentioned steel billet determination method, the continuous casting conditions are changed so that the steel billet does not generate cross-sectional defects. For example, it is preferably that the roll gap between the rolls of the soft reduction zone 42 of the continuous casting apparatus 100 is narrower than the thickness of the ingot 24, and a soft reduction with a reduction speed of 0.2 mm / min or more and 1.0 mm / min or less is applied to suppress cross-sectional defects of the manufactured steel billet. This soft reduction is preferably applied within a range where the solid phase ratio at the center of the width and the center of the thickness of the ingot 24 in the casting direction is greater than 0.0 and less than or equal to 1.0. The soft reduction can also be applied within a range where the solid phase ratio is greater than 0.0 and less than or equal to 0.7 (flow limit solid phase ratio). Thereby, it can prevent the steel billet 38 from generating cross-sectional defects. In this way, by changing the continuous casting conditions to manufacture the steel billet 38 without cross-sectional defects, even if an oxide film with a high liquid phase ratio is generated in the high-temperature environment in the heating furnace, it can still prevent lamination from occurring when the steel billet is hot-rolled to manufacture a hot-rolled steel sheet. On the other hand, in the determination method of the steel billet of this embodiment, when the steel billet is determined to have no laminating risk, even if the steel billet has a cross-sectional defect, the hot-rolled steel sheet manufactured by hot-rolling the steel billet will not laminate. Therefore, for the steel billet determined to have no laminating risk in the above-mentioned steel billet determination method, it is preferable to expand the roll gap of the guide roll 28 in the soft reduction zone 42 to the thickness of the ingot 24 and change the continuous casting conditions without applying soft reduction to the ingot 24. Thereby, it can suppress the reduction of the life of the guide roll or the roll set in the soft reduction zone 42, and thus can suppress an increase in the manufacturing cost of the steel billet 38. As described above, even when manufacturing a hot-rolled steel sheet using the steel billet manufactured by the manufacturing method of the steel billet of this embodiment, the steel sheet will not laminate. Therefore, the steel billet can be heated in the heating furnace and then hot-rolled using a rolling mill, thereby manufacturing a hot-rolled steel sheet that will not laminate during rolling or at the customer end. Next, the above formula (1) used to obtain the liquid phase ratio index f of the oxide film will be described. In the high-temperature environment in the heating furnace, for a steel slab with a relatively high liquid phase ratio of the oxide film, the frequency of laminar cracking occurs more frequently when manufacturing a hot-rolled steel sheet by hot-rolling the steel slab. Since the oxygen diffusion rate of the liquid-phase oxide film is faster than that of the solid-phase oxide film, it will fully diffuse and oxidize within the cross-sectional defects. When the liquid-phase oxide film is stretched during hot rolling, laminar cracking will occur during rolling or at the customer's end of the hot-rolled steel sheet. Also, according to the investigation by the inventors, it has been confirmed that the liquid phase ratio of the oxide film is particularly high when fayalite (Fe 2 SiO 4 ) is formed, and with the addition of Ti, the liquid phase ratio of the oxide film will be further increased. The formula (1) for obtaining the liquid phase ratio index f of the oxide film is a regression equation formed by considering the degree of influence of the component concentration of the molten steel 18 on the liquid phase ratio of the above oxide film. This regression equation first uses thermodynamic calculations to calculate the liquid phase ratio of the oxide film in the high-temperature environment of a predetermined heating furnace, and then considers the component dependence of this liquid phase ratio to determine the basic regression equation and each parameter. The closer the composition of the FeO-SiO 2 -MnO-TiO 2 system oxide film is to fayalite (Fe 2 SiO 4 ), the lower the melting point. Therefore, the higher the Si concentration of the molten steel 18, the easier it is to form a low-melting-point oxide film. On the other hand, the more the oxidized added elements C and Mn, the less likely it is to form SiO 2 . Therefore, the parameters of the C concentration and Mn concentration in the regression equation become negative values. Since the addition amount of Mn is relatively large, the oxide film formed due to its ratio with Si is switched to be mainly composed of MnO. To consider this point, the term of Si concentration / Mn concentration is introduced into the regression equation. Also, Ti has the effect of lowering the melting point of fayalite. Therefore, the Ti concentration is also introduced into the regression equation in the form of multiplying all terms. After determining the regression equation and each parameter based on these considerations, a steel sheet manufactured by heating to 1200 °C in a heating furnace and hot-rolling is used to compare with the actual laminar cracking occurrence rate. Then, it is normalized so that the boundary between the liquid phase ratio index f of the oxide film that does not cause laminar cracking and the liquid phase ratio index f of the oxide film that causes laminar cracking becomes 1.00. The example of the regression equation determined in this way is the above formula (1), and the example of the parameter at this time is A Ti1 : 8.2, A C : -0.7, A Si : 2.0, A Mn : -0.5, A Ti2 : 23, A Si / Mn : 2.7. The above examples show examples of calculating the liquid phase ratio of the oxide film using thermodynamic calculations and determining the regression equation and various parameters for obtaining the liquid phase ratio index f of the oxide film. However, the method for determining the regression equation and various parameters for obtaining the liquid phase ratio index f of the oxide film is not limited to this. For example, it is also possible to actually form the oxide film in a laboratory experiment and determine the regression equation and various parameters by investigating the state of the oxide film. Also, it is possible to determine the regression equation and various parameters based on the relationship between the occurrence rate of mid-layer crack defects in an actual machine and the component concentration of molten steel 18. The method for determining the steel billet of this embodiment shows an example of determining the risk of mid-layer cracks of the steel billet manufactured using the continuous casting apparatus 100 by using the component concentration of the molten steel 18 injected into the mold 10 of the continuous casting apparatus 100. However, it is not limited to this. The method for determining the steel billet of this embodiment can not only determine the steel billet manufactured using the continuous casting apparatus 100, but also determine the risk of mid-layer cracks of the steel billet even if it is a steel billet manufactured in the past. In this case, as long as the component concentration of the molten steel 18 injected into the mold 10 is replaced with the component concentration of the steel billet manufactured in the past, the liquid phase ratio index of the oxide film generated by the steel billet in the high-temperature environment in the heating furnace can be obtained. The embodiments of the present invention are not limited to the above embodiments, and various modifications can be added. The method for manufacturing the steel billet of this embodiment shows an example of changing the continuous casting conditions and applying soft reduction to the steel billet 38 determined to have a risk of mid-layer cracks in the second step. However, it is not limited to this. It is also possible to determine whether the steel billet 38 has cross-sectional defects for the steel billet 38 determined to have a risk of mid-layer cracks in the second step, and weld the cross-sectional defects of the steel billet 38 determined to have cross-sectional defects. Whether the steel billet 38 has cross-sectional defects can be determined according to the following two methods for determining cross-sectional defects. <Determination Method 1 of Cross-Sectional Defects> First, the determination method 1 of the cross-sectional defects of the steel billet will be described. Through in-depth research by the inventors, it was found that if the temperature difference in the width direction of the ingot 24 at the end of solidification becomes large, the steel billet 38 manufactured by cutting the ingot 24 is likely to have cross-sectional defects. The temperature difference in the width direction of the ingot 24 is measured within the range from the position where the ingot support roll 32 is not provided to the position where the ingot cutting machine 36 is provided in the casting direction. Figure 3 is a chart showing the relationship between the temperature difference in the width direction and the cross-sectional defects of the steel billet. In Figure 3, the horizontal axis represents the width dimension (mm) of the ingot, 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 highest temperature within the range of ±100 mm from the center in the width direction of the ingot 24 and the highest temperature in the entire width direction of the ingot (excluding the range of ±100 mm from the center in the width direction). The "〇" in the chart represents a manufacturing example of the steel billet 38 without cross-sectional defects, and the "×" represents a manufacturing example of the steel billet 38 with cross-sectional defects confirmed. As shown in Figure 3, if the temperature difference in the width direction of the ingot 24 is 80 °C or more, the steel billet 38 is likely to have cross-sectional defects; if the temperature difference in the width direction of the ingot 24 is 100 °C or more, the steel billet 38 has cross-sectional defects. From this result, it can be known that by using the temperature difference in the width direction of the ingot 24 within the range from the completion of solidification of the ingot 24 to the position where the ingot cutting machine 36 is provided, it is possible to determine whether the steel billet 38 has cross-sectional defects. Specifically, when the steel billet determined to have the risk of laminar cracking in the second step is used, the infrared thermal imager 44 is used to measure the temperature difference in the width direction of the ingot 24. When the temperature difference exceeds 80 °C, it is determined that the manufactured steel billet 38 has cross-sectional defects. On the other hand, when the temperature difference is 80 °C or less, it is determined that the manufactured steel billet 38 does not have cross-sectional defects. The temperature difference of 80 °C is an example of the threshold value of the predetermined temperature difference, and it is determined by investigating the relationship between the temperature difference in the width direction and the presence or absence of cross-sectional defects through the chart shown in Figure 3. In this way, the manufacturing method of the steel billet of the present embodiment can determine whether there are cross-sectional defects for the steel billet determined to have the risk of laminar cracking in the second step by using the determination method 1 of cross-sectional defects. <Determination Method 2 of Cross-Sectional Defects> Next, the determination method 2 of the cross-sectional defects of the steel billet will be described. The determination method 2 of cross-sectional defects is to use the image data generated by photographing the cutting surface cut by the ingot cutting machine 36 to determine whether the steel billet 38 has cross-sectional defects. If the steel billet 38 has cross-sectional defects, cracks (crazing) will occur on the cutting surface of the steel billet 38. Therefore, by using the image data generated by photographing the cutting surface of the steel billet 38 to detect the cracks on the cutting surface, it is possible to determine whether the steel billet 38 has cross-sectional defects. The detection of cracks can be implemented by an operator identifying the image data, or by performing image processing on the image data to detect cracks. Since the brightness of the cracks is lower than that of other parts, for example, by using a threshold value that can distinguish the brightness between the cracks and other parts, the image data is binarized, and the width dimension and height dimension of the cracks are obtained. By comparing these dimensions with the actual dimensions of the cracks in the past, it is possible to detect the presence or absence of cracks. The photographing device 48 preferably has an infrared cut filter. Since the cutting surface of the steel slab 38 is red-hot, if the cutting surface is directly photographed, the difference in pixel brightness between the crack part and the pixel brightness of other parts becomes small, and it becomes difficult to detect the crack. In contrast, by using a photographing device equipped with an infrared cut filter that blocks light with a wavelength of 700 nm or more, the influence caused by red heat can be reduced, and the crack on the cutting surface can be stably detected. In addition, it is preferable to illuminate the cutting surface of the steel slab 38 by the lighting device 46. Moreover, it is preferable that the lighting device 46 illuminates the cutting surface of the steel slab 38 obliquely from either the upper or lower direction in the thickness direction of the steel slab 38. In this way, by illuminating the cutting surface obliquely from the lighting device 46, a shadow can be formed on the crack, and the crack can be detected more stably. If the steel slab determined to have the risk of laminar cracking in the second step has a cross-sectional defect, the possibility of laminar cracking occurring in the hot-rolled steel sheet manufactured by hot rolling becomes high. Therefore, when it is determined to have a cross-sectional defect by the cross-sectional defect determination method 1 or 2, the crack generated in the cutting surface of the steel slab 38 is welded to close the gap. Thereby, it is possible to prevent the oxide film with a high liquid phase ratio from entering the gap of the cross-sectional defect. Therefore, even for a steel slab determined to have the risk of laminar cracking and having a cross-sectional defect, it is still possible to prevent laminar cracking from occurring in the hot-rolled steel sheet manufactured by hot rolling. On the other hand, when it is determined by the cross-sectional defect determination method 1 or 2 that there is no cross-sectional defect, the hot-rolled steel sheet manufactured using the steel slab 38 will not have laminar cracking. Therefore, the steel slab 38 determined to have no cross-sectional defect by the cross-sectional defect determination method 1 or 2 can be directly used for the manufacture of hot-rolled steel sheets. In this way, by making a determination using the cross-sectional defect determination method 1 or 2, it is possible to reduce the removal of the steel slab from the manufacturing production line for welding the cutting surface of the steel slab 38, and thus the increase in the manufacturing cost of the steel slab can be suppressed. [Embodiment] Hereinafter, an embodiment in which the effects of the present invention have been confirmed will be described. For steel grades A to C, steel slabs are manufactured using the continuous casting device 100. The component compositions of steel grades A to C, the liquid phase ratio index f of the oxide film, the presence or absence of soft reduction, and the occurrence rate of laminar cracking are shown in Table 1 below. The liquid phase ratio index f of the oxide film is a value 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}...(2) [Table 1] In Table 1, the reason for conducting multiple tests using the same steel grade is to evaluate the deviation of the component composition. In Table 1, the laminar tearing occurrence rate represents the proportion (%) of laminar tearing occurring in the hot-rolled steel sheets manufactured using each steel slab. Test numbers No.1 to 3, 5 to 7 represent the appropriateness of the determination of the liquid-phase rate index f of the oxide film due to component deviation. As shown in Table 1, if a steel slab with a liquid-phase rate index f of the oxide film of 1.00 or more is used, laminar tearing will occur in the hot-rolled steel sheet manufactured by heating the steel slab to 1200 °C in a heating furnace and then hot-rolling. On the other hand, if a steel slab with a liquid-phase rate index f of the oxide film less than 1.00 is used, laminar tearing will not occur in the hot-rolled steel sheet manufactured by heating the steel slab to 1200 °C in a heating furnace and then hot-rolling. Based on this result, the component concentration of the molten steel before casting using a continuous casting machine is used to calculate the liquid-phase rate index f of the oxide film of the steel slab manufactured from this molten steel. Then, it is confirmed that by determining whether the calculated index f is above a predetermined threshold value, the laminar tearing risk of the hot-rolled steel sheet manufactured by hot-rolling the steel slab can be determined. Test numbers No.4, 8, 9 represent the results of changing the casting conditions and performing soft reduction on steel slabs with a liquid-phase rate index f of the oxide film of 1.00 or more and evaluated as having a laminar tearing risk. Although test numbers No.4, 8, 9 are evaluated as having a laminar tearing risk, soft reduction is applied and the cross-sectional defects of the steel slab disappear. The reduction speeds for applying soft reduction are No.4: 0.24 mm / min, No.8: 0.66 mm / min, No.9: 0.96 mm / min. In this way, for the steel slabs of No.4, 8, 9 to which soft reduction is applied, laminar tearing does not occur in the hot-rolled steel sheets manufactured by heating the steel slab to 1200 °C in a heating furnace and then hot-rolling. Based on this result, it is confirmed that when a steel slab determined to have a laminar tearing risk is subjected to appropriate soft reduction at the end of solidification, the laminar tearing of the hot-rolled steel sheet manufactured by hot-rolling the steel slab can be suppressed. 10: Mold 12: Feeding trough 14: Slide nozzle 16: Immersion nozzle 18: Molten steel 20: Solidified shell 22: Unsolidified layer 24: Cast sheet 26: Support roll 28: Guide roll 30: Pinch roll 32: Cast sheet support roll 34: Conveyor roll 36: Cast sheet cutting machine 38: Steel slab 40: Complete solidification position 42: Soft reduction zone 44: Infrared thermal imager 46: Lighting device 48: Photographing device 100: Continuous casting device Figure 1 is a side schematic view of a continuous casting device for implementing the determination method of a steel slab and the manufacturing method of a steel slab according to this embodiment. Figure 2 is a chart showing the relationship between the liquid-phase rate index f of the oxide film and the occurrence rate of laminar tearing. Figure 3 is a chart showing the relationship between the temperature difference in the width direction and the cross-sectional defects of the steel slab.

Claims

1. A method for determining a steel billet, which is a steel billet containing Ti, C, Si and Mn as components and manufactured using a continuous casting apparatus; comprising: a first step of determining the liquid phase ratio of the oxide film generated on the steel billet by using the component concentration of the molten steel injected into the mold of the continuous casting apparatus; and a second step of determining the steel billet as having a risk of delamination when the liquid phase ratio of the oxide film is above a predetermined threshold; wherein the liquid phase ratio of the oxide film is determined by the Ti concentration, C concentration, Si concentration and Mn concentration of the molten steel.

2. The method for determining the steel billet as requested in item 1, wherein, The liquid phase ratio of the oxide film is determined by the following formula (1): f=ATi1×[Ti]×{AC×[C]+ASi×[Si]+AMn×[Mn]+ATi2×[Ti]+ASi / Mn×([Si] / [Mn])0.5}・・・(1) In the above formula (1), f is the liquid phase ratio of the oxide film (-), [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 (mass%) of the molten steel, and ATi1, AC, ASi, AMn, ATi2 and ASi / Mn are parameters.

3. The method for determining the steel billet as requested in item 1 or 2, wherein, In step 2 above, when the liquid phase ratio of the oxide film does not reach the predetermined threshold, the steel billet is determined to be a steel billet without the risk of delamination.

4. A method for manufacturing a steel billet, wherein the steel billet is determined to be of risk of delamination in the steel billet determination method of claim 1 or 2, and the solid fraction of the center of the width and the center of the thickness of the casting sheet in the casting direction is greater than 0.0 and less than 1.0, the roll opening of the pressing roll of the continuous casting device is changed and the casting sheet is lightly pressed down.

5. A method for manufacturing a steel billet, wherein the steel billet is manufactured using a continuous casting apparatus, and when the steel billet is determined to be at risk of delamination in the steel billet determination method of claim 1 or 2, the temperature difference in the width direction of the cast sheet is measured and the presence or absence of cross-sectional defects is determined within the range from the position where the casting sheet support roller is never installed to the position where the casting sheet is cut.

6. A method for manufacturing a steel billet, wherein the steel billet is manufactured using a continuous casting apparatus, and when the steel billet is determined to be at risk of delamination in the steel billet determination method of claim 1 or 2, the cut surface of the steel billet is photographed to generate image data, and the image data is used to determine whether there are cross-sectional defects.

7. The method for manufacturing the steel billet as described in claim 6, wherein the cut surface is photographed using a photographing device equipped with an infrared cutoff filter.

8. The method of manufacturing a steel billet as described in any of claims 5 to 7, wherein the steel billet is determined to have a cross-sectional defect by welding the gap of the cross-sectional defect.

9. A method for manufacturing a steel plate, which is a method for manufacturing a steel plate by heating and hot rolling a steel billet, wherein the steel billet is determined to have no risk of delamination in the method for determining the steel billet of claim 3.

10. A method for manufacturing a steel plate, which is a method for manufacturing a steel plate by heating and hot rolling a steel billet, wherein the steel billet manufactured in the method for manufacturing the steel billet of claim 4 is heated and hot rolled.

11. A method for manufacturing a steel plate, which is a method for manufacturing a steel plate by heating and hot rolling a steel billet, wherein the steel billet is determined to be free of the aforementioned cross-sectional defects in the method for manufacturing the steel billet of any one of claims 5 to 7.

12. A method for manufacturing a steel plate, which is a method for manufacturing a steel plate by heating and hot rolling a steel billet, wherein the steel billet manufactured in the method for manufacturing the steel billet of claim 8 is heated and hot rolled.

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