Method for manufacturing hot-rolled steel sheets, method for predicting the temperature history of hot-rolled steel sheets, and method for predicting the hardening portion of hot-rolled steel sheets

Abstract

The present invention aims to provide a method for manufacturing a hot-rolled steel sheet capable of predicting the temperature history of irregularities on the end surface of a coil. A method for manufacturing a hot-rolled steel sheet according to one aspect of the present invention comprises: a process of measuring the surface temperature of a hot-rolled strip-shaped steel material; a process of calculating the temperature history in a cooling state after winding, assuming that the strip-shaped steel material is wound into a coil shape without irregularities on the end surface, based on the surface temperature measured in the measurement process; a process of actually winding the strip-shaped steel material into a coil shape after the measurement process; a process of scanning the end surface of the coil wound in the winding process with a displacement meter and deriving the size of the irregularities on the end surface over the radius of the coil; and a process of predicting the temperature history of the irregularities in a cooling state using the temperature history calculated in the calculation process and the size of the irregularities derived in the derivation process.

Description

Technology Field

[0001] The present invention relates to a method for manufacturing a hot-rolled steel sheet, a method for predicting the temperature history of a hot-rolled steel sheet, and a method for predicting the hardening portion of a hot-rolled steel sheet. Background Technology

[0002] Hot-rolled steel sheets are manufactured by winding hot-rolled strip-shaped steel into a coil shape and cooling the coil to room temperature. This hot-rolled steel sheet is then re-formed into a strip shape, pickled, and cold-rolled to become a cold-rolled steel sheet. A manufacturing problem associated with this cold-rolled steel sheet is the breakage of the steel during the passing process. If the steel breaks, it is necessary to stop the passing line to perform repair work, which increases repair costs and reduces production efficiency. Furthermore, the breakage of the steel can also be a cause of equipment failure. Prior art literature

[0003] Japanese Patent Publication No. 2014-593 Japanese Patent Publication No. 2010-112958 The problem to be solved

[0004] One of the causes of fracture during cold rolling of steel with high hardenability is cracking at the ends of the steel. If cracks form at the ends of the steel during cold rolling, stress concentrates at these cracked areas during rolling, causing the cracks to grow and easily leading to fracture.

[0005] Defects in the winding shape of hot-rolled coils can be cited as a cause of these cracks. Specifically, if there are winding defects in the coil after hot rolling, the irregularities on the coil's end surface increase. If there are large convexities on the coil's end surface, these convexities function as pins, accelerating the cooling rate of the convexities during the coil's cooling process. As the cooling rate of the convexities accelerates, hard phases such as bainite or martensite tend to become mixed within them. Consequently, voids form during the cold rolling of the steel, and the growth of these voids makes it easy to cause cracks at the ends.

[0006] In this regard, the inventors have discovered that by predicting the temperature history of the irregularities (particularly the convex parts) on the end surface of the coil, the possibility of steel fracture in subsequent processes can be known in advance.

[0007] In addition, Patent Document 1 describes predicting temperature non-uniformity on a run-out table (ROT) of a hot-rolled steel sheet and controlling the manufacturing conditions of the rolled steel sheet before it is wound into a coiler so that the predicted temperature non-uniformity is reduced.

[0008] In addition, Patent Document 2 describes a technique for identifying whether the measured value of the coil end is a tongue shape or a telescopic shape when calculating the telescopic amount of the coil end surface by scanning a distance meter across the coil diameter on the end surface of a metal plate coil and measuring the distance between the distance meter and the coil end surface. Patent Document 2 describes determining that a coil in which the difference in the measured distance between the innermost metal plates at both ends of the coil diameter, or the difference in the measured distance between the outermost metal plates at both ends of the coil diameter, each exceeds a threshold value is measuring the tongue shape of the coil end of the metal plate, and calculating the telescopic amount of the coil by excluding the innermost and / or outermost distance data in which the tongue shape was measured.

[0009] However, in patent documents 1 and 2, the relationship between the irregularities on the coil end surface and cracks in the coil end is not examined.

[0010] The present invention is made in consideration of the circumstances described above and aims to provide a method for manufacturing a hot-rolled steel sheet and a method for predicting the temperature history of the unevenness on the end surface of a coil, which can predict the temperature history of the unevenness. Furthermore, the present invention aims to provide a method for predicting the hardening portion of a hot-rolled steel sheet, which can predict whether or not the steel material will fracture in a subsequent process based on the temperature history of the unevenness on the end surface of the coil. means of solving the problem

[0011] A method for manufacturing a hot-rolled steel sheet according to one aspect of the present invention comprises: a measurement process for measuring the surface temperature of a hot-rolled strip-shaped steel material; a first calculation process for calculating the temperature history in a cooling state after winding, assuming that the strip-shaped steel material is wound into a coil shape without irregularities on the end surface, based on the surface temperature measured in the measurement process; a winding process for actually winding the strip-shaped steel material into a coil shape after the measurement process; a derivation process for scanning the end surface of the coil wound in the winding process with a displacement meter and deriving the size of the irregularities on the end surface over the radius of the coil; and a first prediction process for predicting the temperature history of the irregularities in a cooling state using the temperature history calculated in the first calculation process and the size of the irregularities derived in the derivation process.

[0012] The method for manufacturing the hot-rolled steel sheet can predict the temperature history of the uneven surface of the end surface of the coil formed by winding the strip-shaped steel material during the cooling state.

[0013] The method for manufacturing the hot-rolled steel sheet may further comprise a second calculation process for calculating a phase transformation rate using the temperature history predicted in the first prediction process, and a second prediction process for predicting the hardened portion of the strip-shaped steel using the phase transformation rate calculated in the second calculation process. By comprising the second calculation process and the second prediction process, the method for manufacturing the hot-rolled steel sheet can predict whether or not the steel will break in a subsequent process.

[0014] In the above derivation process, it is preferable to determine the size of the irregularities based on the median value of the measurements taken by the displacement gauge. In this way, by determining the size of the irregularities based on the median value of the measurements taken by the displacement gauge in the above derivation process, it is easy to easily and accurately predict the temperature history of the irregularities on the end surface of the coil under a cooling state.

[0015] In the above derivation process, it is preferable to determine the size of the irregularities using a two-dimensional coordinate system defined by the protrusion direction of the irregularities and the scanning direction by the displacement gauge. In this way, by determining the size of the irregularities using a two-dimensional coordinate system defined by the protrusion direction of the irregularities and the scanning direction by the displacement gauge in the above derivation process, it is easy to easily and accurately predict the temperature history of the irregularities on the end surface of the coil under a cooling state.

[0016] A method for predicting the temperature history of a hot-rolled steel sheet according to another aspect of the present invention comprises: a measurement process for measuring the surface temperature of a hot-rolled strip-shaped steel material; a first calculation process for calculating the temperature history in a cooling state after winding, assuming that the strip-shaped steel material is wound into a coil shape without irregularities on the end surface, based on the surface temperature measured in the measurement process; a winding process for actually winding the strip-shaped steel material into a coil shape after the measurement process; a derivation process for scanning the end surface of the coil wound in the winding process with a displacement meter and deriving the size of the irregularities on the end surface over the radius of the coil; and a first prediction process for predicting the temperature history of the irregularities in a cooling state using the temperature history calculated in the first calculation process and the size of the irregularities derived in the derivation process.

[0017] The method for predicting the temperature history of the hot-rolled steel sheet can predict the temperature history of the uneven surface of the end surface of the coil formed by winding the strip-shaped steel material in a cooling state.

[0018] A method for predicting a hardening portion of a hot-rolled steel sheet according to another aspect of the present invention comprises: a measurement process for measuring the surface temperature of a hot-rolled strip-shaped steel material; a first calculation process for calculating the temperature history in an open-cooling state after winding, assuming that the strip-shaped steel material is wound into a coil shape without irregularities on the end surface, based on the surface temperature measured in the measurement process; a winding process for actually winding the strip-shaped steel material into a coil shape after the measurement process; a derivation process for scanning the end surface of the coil wound in the winding process with a displacement meter and deriving the size of the irregularities on the end surface over the radius of the coil; a first prediction process for predicting the temperature history of the irregularities in an open-cooling state using the temperature history calculated in the first calculation process and the size of the irregularities derived in the derivation process; a second calculation process for calculating a phase transformation rate using the temperature history predicted in the first prediction process; and the phase transformation rate calculated in the second calculation process A second prediction process is provided to predict the hardened portion of the above-mentioned strip-shaped steel using the above method.

[0019] The method for predicting the hardening portion of the hot-rolled steel sheet can predict whether the steel will fracture in subsequent processes based on the temperature history of the irregularities on the end surface of the coil. Effects of the invention

[0020] As described above, the method for manufacturing a hot-rolled steel sheet according to one aspect of the present invention and the method for predicting the temperature history of a hot-rolled steel sheet according to another aspect can predict the temperature history of irregularities on the end surface of a coil. Furthermore, the method for predicting the hardening portion of a hot-rolled steel sheet according to another aspect of the present invention can predict whether or not the steel will fracture in a subsequent process based on the temperature history of irregularities on the end surface of the coil. Brief explanation of the drawing

[0021] FIG. 1 is a flowchart illustrating a method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention. Figure 2 is a schematic diagram illustrating a manufacturing facility for hot-rolled steel sheets used in the method for manufacturing hot-rolled steel sheets of Figure 1. Figure 3 is an explanatory diagram of the sequence of calculating the temperature history of a virtual coil by the first calculation process of the method for manufacturing a hot-rolled steel sheet of Figure 1. Figure 4 is a schematic diagram illustrating the scanning position of the end face of a coil by a displacement gauge in the derivation process of the method for manufacturing a hot-rolled steel sheet of Figure 1. Figure 5 is an explanatory diagram of the sequence of deriving the size of irregularities by the derivation process of the manufacturing method of the hot-rolled steel sheet of Figure 1. Figure 6 is a flowchart illustrating a method for manufacturing a hot-rolled steel sheet that is different in form from the method for manufacturing a hot-rolled steel sheet of Figure 1. Figure 7 is a graph showing the temperature history in a cooling state based on the time immediately after winding the upper surface of the virtual coil by the first production process of No. 1. Figure 8 is a graph showing the measurement result of the shape of the irregularities on the end surface of the coil by the displacement gauge in the No. 1 extraction process and the result of the extraction of the size of the irregularities on the end surface of the coil by the extraction part. Figure 9 is a graph showing the temperature history in a cooling state based on the time immediately after winding of the end face of the coil predicted in the first prediction process of No. 1, and the actual measured value of the temperature of this end face. Figure 10 is a graph showing the temperature history of the upper surface in a cooling state based on the time immediately after winding the virtual coil produced in the first production process of No. 2. Figure 11 is a graph showing the measurement result of the shape of the irregularities on the end surface of the coil by the displacement gauge in the No. 2 extraction process and the result of the extraction of the size of the irregularities on the end surface of the coil by the extraction part. Figure 12 is a graph showing the temperature history in a cooling state based on the time immediately after winding of the end face of the coil predicted in the first prediction process of No. 2, and the measured value of the temperature of this end face. Specific details for implementing the invention

[0022] Hereinafter, embodiments of the present invention will be described in detail with appropriate reference to the drawings.

[0023] [First Embodiment]

[0024] Method for manufacturing hot-rolled steel sheets

[0025] The method for manufacturing a hot-rolled steel sheet of FIG. 1 comprises a process for measuring the surface temperature of a hot-rolled strip-shaped steel material (measurement process S1); a process for calculating the temperature history in a cooling state after winding, assuming that the strip-shaped steel material is wound into a coil shape without irregularities on the end surface, based on the surface temperature measured in the measurement process S1 (first calculation process S2); a process for actually winding the strip-shaped steel material into a coil shape after the measurement process S1 (winding process S3); a process for scanning the end surface of the coil wound in the winding process S3 with a displacement meter and deriving the size of the irregularities on the end surface over the radius of the coil (derivation process S4); and a process for predicting the temperature history in a cooling state of the irregularities using the temperature history calculated in the first calculation process S2 and the size of the irregularities derived in the derivation process S4 (first prediction process S5). A measurement process S1, a first calculation process S2, a winding process S3, a derivation process S4, and a first prediction process S5 constitute a method for predicting the temperature history of a hot-rolled steel sheet according to one aspect of the present invention. In addition, "coil shape" refers to a spiral shape when viewed in the axial direction. "End surface of the coil" refers to a surface perpendicular to the central axis in the coil. That is, "end surface of the coil" refers to a surface formed by the end of the strip-shaped steel material in the width direction.

[0026] According to the method for manufacturing the hot-rolled steel sheet, by predicting the temperature history of the irregularities on the end surface of the coil, the possibility of fracture of the steel material when manufacturing a cold-rolled sheet using this hot-rolled steel sheet can be known in advance.

[0027] When describing the method for manufacturing the hot-rolled steel sheet, first, with reference to FIG. 2, a manufacturing facility (1) for the hot-rolled steel sheet capable of carrying out the method for manufacturing the hot-rolled steel sheet (hereinafter also briefly referred to as "manufacturing facility (1)") will be described.

[0028] [Manufacturing Equipment for Hot-Rolled Steel Sheets]

[0029] The manufacturing facility (1) of FIG. 2 has a plurality of pairs of rolling rolls (2a), a conveying unit (2b) for conveying a strip-shaped steel material (X) hot-rolled by these rolling rolls (2a), and a winder (2c) for winding the strip-shaped steel material (X) conveyed to the conveying unit (2b) into a coil shape, a hot rolling device (2) constituting a hot rolling line, a measuring device (3) for measuring the surface temperature of the strip-shaped steel material (X) being conveyed to the conveying unit (2b), a calculation device (4) for calculating the temperature history in a cooling state of a coil (virtual coil) assuming that the strip-shaped steel material (X) is wound into a coil shape without irregularities on the end surface, based on the surface temperature of the strip-shaped steel material (X) measured by the measuring device (3), and the size of the irregularities on the end surface (E) of the coil (X1) wound by the winder (2c). The manufacturing facility (1) is equipped with a drawing device (5) that forms a drawing line that is drawn over the radius of a coil (X1), and a prediction device (6) that predicts the temperature history of the irregularities in a cooling state using the temperature history calculated by the drawing device (4) and the size of the irregularities drawn by the drawing device (5). In addition, the manufacturing facility (1) is equipped with a cooling device (7) that cools the coil (X1) after it passes through the drawing line. Furthermore, the manufacturing facility (1) may further be equipped with a cold rolling device that cold-rolls the coil (X1) after it has been cooled by the cooling device (7), an annealing device that anneales the strip-shaped steel material after it has been cold-rolled by the cold rolling device, etc.

[0030] The hot rolling device (2) performs rough rolling and finish rolling on a steel plate heated in a heating furnace (not shown), and then conveys the strip-shaped steel material (X) after rolling to a coiler (2c) by a conveying unit (2b), and winds it into a coil shape in the coiler (2c). The conveying unit (2b) has, for example, a plurality of conveying rollers.

[0031] The measuring device (3) has a non-contact temperature sensor (3a), such as a thermograph. The measuring device (3) measures the surface temperature of the strip-shaped steel material (X) after hot rolling and before being wound onto the coiler (2c). The measuring device (3) measures the temperature of the entire surface area (full length and full width) of the strip-shaped steel material (X).

[0032] The calculation device (4) is configured, for example, as a computer. The calculation device (4) assumes, for example, that a coil (virtual coil) without irregularities on the end surface formed by winding a strip-shaped steel material (X) is cylindrical, and calculates the temperature history of this virtual coil in a cooled state using a two-dimensional model in a polar coordinate system.

[0033] The extraction device (5) has a conveyor (5a) that transports a coil (X1) wound on a winder (2c), a displacement meter (5b) that scans the end surface (E) of the coil (X1) being transported on the conveyor (5a) and measures the shape of the end surface (E), and an extraction unit (5c) that derives the size of the irregularities of the end surface (E) of the coil (X1) based on the shape measured by the displacement meter (5b). The displacement meter (5b) measures the shape of the end surface (E) of the coil (X1) over the radius of the coil (X1), and preferably over the diameter. For example, a laser displacement meter is used as the displacement meter (5b). The displacement meter (5b) has a laser irradiation unit that irradiates laser light onto the end surface (E) of the coil (X1), and a light receiving element that receives a portion of the light reflected from the end surface (E). The displacement meter (5b) reads the reflected light of the laser light irradiated onto the end surface (E) from the laser irradiation unit using the light receiving element. The displacement meter (5b) is configured to measure the shape of the end surface (E) of the coil (X1) by a triangulation method. The output unit (5c) is configured, for example, as a computer. The displacement meter (5b) and the output unit (5c) may be configured as a single unit.

[0034] The prediction device (6) is configured, for example, as a computer. The prediction device (6) predicts the temperature history of the unevenness of the end surface (E) of the coil (X1) in the case where it is assumed that the coil (X1) actually wound by the winder (2c) has been cooled by the cooling device (7), etc.

[0035] The cooling device (7) cools the coil (X1) after the shape of the end surface (E) is measured in the extraction device (5). In the manufacturing facility (1), the coil (X1) after being wound by the winder (2c) is heated to about 500°C or higher. The cooling device (7) air-cools the heated coil (X1) to room temperature. In order to cool the wound coil (X1) wound by the winder (2c) in the manufacturing facility (1), if a large convex portion exists on the end surface (E) of the coil (X1), the cooling speed of this convex portion is likely to be faster than other parts.

[0036] [Strip-shaped steel]

[0037] A strip-shaped steel (X) is formed by heating a slab and hot rolling it. The strip-shaped steel (X) has a composition, for example, carbon, silicon, manganese, phosphorus, sulfur, chromium, nickel, molybdenum, and copper, with the remainder being iron and unavoidable impurities. When cold rolling is performed on the strip-shaped steel (X), the coiling temperature in the coiling process S3 can be set to be higher than Ms (martensite transformation start temperature) of the strip-shaped steel (X).

[0038] As for the upper limit of the carbon equivalent (Ceq) shown in the following formula (1) of the strip-shaped steel (X), 0.75% is preferred, and 0.70% is more preferred. If the carbon equivalent (Ceq) of the strip-shaped steel (X) exceeds the upper limit, the risk of the formation of a martensite phase increases when the cooling rate during air cooling is high. On the other hand, the lower limit of the carbon equivalent (Ceq) is not particularly limited, but can be 0.55% for example. If the carbon equivalent (Ceq) does not reach the lower limit, the transformation is generally completed by the coiling process S3, so the martensite phase is unlikely to form, and the risk of causing edge splitting in the coil (X1) in the subsequent process is low. Therefore, the method for manufacturing the hot-rolled steel sheet is preferably used when the carbon equivalent (Ceq) of the strip-shaped steel (X) is greater than or equal to the lower limit.

[0039] Ceq[%]=[C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14… (1)

[0040] However, [C], [Si], [Mn], [Ni], [Cr], [Mo], and [V] represent the content (mass %) of C, Si, Mn, Ni, Cr, Mo, and V, respectively.

[0041] (Measurement process)

[0042] The measurement process S1 is performed by a measuring device (3). In the measurement process S1, the surface temperature of the strip-shaped steel (X) after hot rolling and before being wound onto a coiler (2c) is measured over the entire surface area (full length and full width) of the strip-shaped steel (X).

[0043] (1st Output Process)

[0044] The first calculation process S2 is executed by a calculation device (4). In the first calculation process S2, for example, a coil (virtual coil) without irregularities on the end surface formed by winding a strip-shaped steel material (X) is assumed to be cylindrical, and the temperature history of this virtual coil in a cooling state is calculated by a two-dimensional model in a polar coordinate system. The first calculation process S2 may be executed before the winding process S3, or it may be executed after the winding process S3. Additionally, it is possible to execute the first calculation process S2 after the derivation process S4.

[0045] Referring to FIG. 3, an example of the calculation sequence of the temperature history of a virtual coil (X2) in a cooled state by the first calculation process S2 is described. In the first calculation process S2, the temperature history of the virtual coil (X2) in a cooled state is calculated using a two-dimensional model of a polar coordinate system in which the coordinates of the intersection point of a virtual plane containing one end surface (top surface in FIG. 3) of the virtual coil (X2) and the center axis of the virtual coil (X2) are set as the origin (O(0, 0)), the coordinates in the center axis direction relative to the origin (O) are z[m], and the coordinates in the diameter direction relative to the origin (O) are r[m]. In the first calculation process S2, a plurality of calculation points are provided in the center axis direction and the radial direction of the virtual coil (X2), respectively, and the temperature history in a cooled state is calculated for each calculation point. Specifically, in the first calculation process S2, the temperature history of the virtual coil (X2) in a cooled state is calculated by using the following equation (2) for the calculation point inside the virtual coil (X2) (the calculation point of the part not exposed to the outside air) and the following equation (3) for the calculation point of the part exposed to the outside air, with the temperature after time t at the calculation point of the virtual coil (X2) based on the time immediately after winding being Φ[℃].

[0046] [Number 1]

[0047]

[0048] [Number 2]

[0049]

[0050] In addition, in the above equations (2) and (3), H: enthalpy [kcal / kg], ρ: density of the part corresponding to the calculation point of the strip-shaped steel [kg / m³], λ r : Thermal conductivity in the radial direction [kcal / m / hr / ℃], λ z : Axial thermal conductivity [kcal / m / hr / ℃], ε: Emissivity [-], σ: Stefan-Boltzmann constant [kcal / m² / hr / ℃] 4 ], F12 : Shape factor [-], α: Natural convection heat transfer rate [kcal / hr / m² / ℃], V: Volume of the part corresponding to the calculation point of the strip-shaped steel [m³], A: Surface area of ​​the part corresponding to the calculation point of the strip-shaped steel [m²]. In addition, in the above Equation (3), q is a boundary condition. This boundary condition is applied to the inner surface of the virtual coil (X2) by the following Equation (4), and to the end surface and outer surface of the virtual coil (X2) by the following Equation (5). In the following Equations (4) and (5), T: Surface temperature [℃] measured in the measurement process S1 of the part corresponding to the calculation point, T f : Refers to the ambient temperature [°C] during cooling.

[0051] [Number 3]

[0052]

[0053] [Number 4]

[0054]

[0055] (Winding process)

[0056] In the winding process S3, the strip-shaped steel (X), after the surface temperature is measured in the measurement process S1, is wound into a coil shape by a winder (2c) at a high temperature. In the winding process S3, the winding temperature is preferably higher than the Ms temperature of the strip-shaped steel (X) to prevent the formation of martensite phases. As a lower limit of the winding temperature, 400°C is preferred, 500°C is more preferred, and 560°C is more preferred. Meanwhile, as an upper limit of the winding temperature, 700°C is preferred, and 670°C is more preferred. If the winding temperature does not reach the lower limit, the strength of the strip-shaped steel (X) becomes too high, and there is a risk that the load on the equipment will increase in subsequent processes such as the cold rolling process. Conversely, if the winding temperature exceeds the upper limit, there is a risk that the scale thickness on the surface of the strip-shaped steel (X) will increase. In addition, "winding temperature" refers to the surface temperature of the strip-shaped steel (X) immediately before winding.

[0057] (Derivation process)

[0058] The extraction process S4 is executed by the extraction device (5). As illustrated in FIGS. 2 and 4, in the extraction process S4, the end surface (E) of the coil (X1) being transported on the conveyor (5a) is scanned by the displacement meter (5b), and the shape of the irregularity of the end surface (E) is measured over the radius of the coil (X1), preferably over the diameter. In addition, in the extraction process S4, the size of the irregularity of the end surface (E) is determined by the extraction unit (5c) using a two-dimensional coordinate system defined by the protrusion direction of the irregularity (the direction of the center axis of the coil (X1)) and the scanning direction by the displacement meter (5b) (the direction of the radius of the coil (X1)). The method for manufacturing the hot-rolled steel sheet in question makes it easy and accurate to predict the temperature history of the unevenness of the end surface (E) of the coil (X1) in a cooling state by using the above two-dimensional coordinate system to determine the size of the unevenness of the end surface (E), by the first prediction process S5 described later.

[0059] In the extraction process S4, it is preferable to determine the size of the irregularity of the end surface (E) based on the median value of the measurement value by the displacement gauge (5b). Specifically, in the extraction process S4, it is preferable to continuously measure the end surface (E) of the coil (X1) over a radius using the displacement gauge (5b), and then determine the size of the irregularity of the end surface (E) based on the median value of the measurement value by the displacement gauge (5b) using the extraction unit (5c). By determining the size of the irregularity of the end surface (E) based on the median value, the manufacturing method of the hot-rolled steel sheet can appropriately measure the size of the irregularity of the entire coil (X1), even in cases where a large protruding part (telescope) caused by winding misalignment is formed at the end of the outer circumference and / or inner circumference of the coil (X1). As a result, the temperature history of the unevenness of the end surface (E) of the coil (X1) in the cooling state can be easily and accurately predicted by the first prediction process S5.

[0060] Referring to FIGS. 4 and 5, an example of the sequence for deriving the size of the irregularities on the end surface (E) of a coil (X1) by the derivation process S4 is described. First, in the derivation process S4, the end surface (E) of the coil (X1) being transported on the conveyor (5a) is scanned by a displacement meter (5b), and the shape of the irregularities on the end surface (E) is measured over the radius of the coil (X1). Next, a reference plane (R) of the end surface (E) is set based on the median value of the measurement value by the displacement meter (5b). Subsequently, the coordinates of the intersection point between the center axis (Z) of the coil (X1) and the reference plane (R) are set as the origin O (0, 0), and the coordinates in the center axis direction relative to the origin (O) are z [m], and the coordinates in the diameter direction relative to the origin (O) are r [m]. Then, the size of the irregularities on the end surface (E) is calculated using a two-dimensional model of a polar coordinate system. Specifically, the coil (X1) is divided into multiple regions in the diameter direction, and the size of the irregularities in each region is averaged. By reflecting this average value in a two-dimensional coordinate system, the size of the irregularities in this region is derived. At this time, it is also possible not to make the lengths of each region in the diameter direction of the coil (X1) equal. For example, to make it easier to reflect irregularities caused by the telescope, the lengths of a pair of regions located at both ends in the diameter direction may be set smaller than other regions. In addition, it is possible to provide a constant threshold value and treat an amount of protrusion below this threshold value as not corresponding to irregularities.

[0061] (1st Forecasting Process)

[0062] The first prediction process S5 is executed by a prediction device (6). In the first prediction process S5, the temperature history of the unevenness of the end surface (E) of the coil (X1) in the cooling state is predicted based on the time immediately after winding. In the first prediction process S5, the temperature history of the unevenness of the end surface (E) in the cooling state is predicted using the above-described equations (2) to (5). At this time, for the reference surface (R), the boundary condition of the above equation (3) is applied by the above equation (5).

[0063] Advantages

[0064] The method for manufacturing the hot-rolled steel sheet can predict the temperature history of the unevenness of the end surface (E) of the coil (X1) formed by winding the strip-shaped steel (X) during the cooling state. Therefore, according to the method for manufacturing the hot-rolled steel sheet, the possibility of steel fracture can be known in advance when manufacturing a cold-rolled sheet using the strip-shaped steel (X).

[0065] The method for predicting the temperature history of the hot-rolled steel sheet can predict the temperature history of the uneven surface (E) of the end surface of the coil (X1) formed by winding the strip-shaped steel (X) in a cooling state. Therefore, according to the method for predicting the temperature history of the hot-rolled steel sheet, the possibility of steel fracture when manufacturing a cold-rolled sheet using the strip-shaped steel (X) can be known in advance.

[0066] [Second Embodiment]

[0067] Method for manufacturing hot-rolled steel sheets

[0068] The method for manufacturing a hot-rolled steel sheet of FIG. 6 comprises: a process for measuring the surface temperature of a hot-rolled strip-shaped steel material (measurement process S11); a process for calculating the temperature history in a cooled state after winding, assuming that the strip-shaped steel material is wound into a coil shape without irregularities on the end surface, based on the surface temperature measured in the measurement process S11 (first calculation process S12); a process for actually winding the strip-shaped steel material into a coil shape after the measurement process S11 (winding process S13); a process for scanning the end surface of the coil wound in the winding process S13 with a displacement meter and deriving the size of the irregularities on the end surface over the radius of the coil (derivation process S14); a process for predicting the temperature history of the irregularities in a cooled state using the temperature history calculated in the first calculation process S12 and the size of the irregularities derived in the derivation process S14 (first prediction process S15); and a first prediction process The method comprises a process for calculating a phase transformation rate (ferrite-pearlite transformation rate) using the temperature history predicted in S15 (second calculation process S16), and a process for predicting the hardened portion of the strip-shaped steel using the phase transformation rate calculated in the second calculation process S16 (second prediction process S17). The measurement process S11, the first calculation process S12, the coiling process S13, the derivation process S14, the first prediction process S15, the second calculation process S16, and the second prediction process S17 constitute a method for predicting the hardened portion of a hot-rolled steel sheet according to one aspect of the present invention. The measurement process S11, the first calculation process S12, the coiling process S13, and the derivation process S14 are omitted from the description because they can be executed in the same order as the measurement process S1, the first calculation process S2, the coiling process S3, and the derivation process S4 of FIG. 1.In addition, in the first calculation process S12, just like in the first prediction process S15 described later, the following equation (6) may be used instead of the above equation (2), and the temperature history may also be calculated using the following equation (7) instead of the above equation (3).

[0069] (1st Forecasting Process)

[0070] In the first prediction process S15, the transformation heat of each time Q calculated in the second calculation process S16 described later. t [kcal / ㎥ / hr] is added to obtain the temperature history of the above-described strip-shaped steel. Specifically, in the first prediction process S15, the following equation (6) is used instead of the above-described equation (2), and the following equation (7) is used instead of the above-described equation (3) to predict the temperature history. The first prediction process S15 can be executed in the same order as the first prediction process S5 of FIG. 1, except that the following equation (6) is used instead of the above-described equation (2) and the following equation (7) is used instead of the above-described equation (3).

[0071] [Number 5]

[0072]

[0073] [Number 6]

[0074]

[0075] (Second Output Process)

[0076] The second calculation process S16 can be executed, for example, by a computer. In the second calculation process (16), the phase transformation rate is calculated from an isothermal transformation equation including the influence of the γ particle size. Additionally, in the second calculation process S16, the transformation heat Q according to the calculated phase transformation rate t...is calculated. Specifically, in the second calculation process S16, the phase transformation rate X[-] is calculated by the following equations (8) and (9), and at the same time, the transformation heat at time t Q is calculated using the following equation (10). t Calculate [kcal / ㎥ / hr]. The transformation calorific value Q calculated in the second calculation process S16. t is used for predicting the temperature history in the first prediction process S15 described above. In addition, this transformation heat generation Q t It may be used to calculate the temperature history in the first output process S12 described above.

[0077] [Number 7]

[0078]

[0079] [Number 8]

[0080]

[0081] [Number 9]

[0082]

[0083] In addition, in the above equations (8) to (10), S: nucleation area term, K: temperature dependency term, Q total : Total transformation heat [kcal / ㎥ / hr], T: Refers to the temperature [°C] of the calculation point calculated in the first calculation process S12 or the first prediction process S15. In addition, a, b, c, m, and n in the above equations (8) to (10) represent constants that are adjusted for each type of steel. These constants can be determined, for example, by creating a TTT line drawing using a hot-rolled crop after defect rolling and adjusting the calculated values ​​to match the experimental values. However, since the transformation rate is affected by the microstructure state before transformation, such as the austenite grain size, it changes according to the hot rolling conditions. Therefore, the value of c is adjusted for each hot rolling condition.

[0084] (Second forecasting process)

[0085] The second prediction process S17 can be executed by a computer, for example. In the second prediction process S17, the hardened portion of the end surface of the coil wound in the winding process S13 is predicted using the phase transformation rate calculated in the second calculation process S16. In the second prediction process S17, for example, the relationship between the phase transformation rate and hardness is determined in advance, and the hardened portion is predicted from the calculated phase transformation rate. In the second prediction process S17, for example, a threshold value of hardness at which fracture may occur in the steel material during a subsequent process such as a cold rolling process may be set in advance, and the presence or absence of fracture of the steel material during the subsequent process may be predicted by comparing this threshold value with the calculated phase transformation rate. In addition, in the second prediction process S17, a threshold value of the phase transformation rate at which fracture may occur in the steel during subsequent processes such as cold rolling may be set in advance, and the presence or absence of fracture in the steel may be predicted by comparing the calculated value with this threshold value.

[0086] Advantages

[0087] The method for manufacturing the hot-rolled steel sheet can predict whether the steel will break in a subsequent process by using the phase transformation rate calculated in the second calculation process S16. According to the method for manufacturing the hot-rolled steel sheet, the risk of the steel breaking during the passing process can be reduced by pre-cutting the hardened portion that causes the breakage in the subsequent process.

[0088] The method for predicting the hardening portion of the hot-rolled steel sheet can predict whether the steel will fracture in the subsequent process by using the phase transformation rate calculated in the second calculation process S16.

[0089] [Other embodiments]

[0090] The above embodiments do not limit the configuration of the present invention. Accordingly, based on the description in this specification and common technical knowledge, the components of each part of the above embodiments may be omitted, substituted, or added, and all thereof should be interpreted as falling within the scope of the present invention.

[0091] For example, in the above derivation process, it is possible to determine the size of the irregularities on the end surface of the coil based on the average value, maximum number of irregularities, etc. However, as described above, in the above derivation process, even if a large protruding part such as a telescope exists, it is preferable to determine the size of the irregularities on the end surface of the coil based on the median value of the measured values ​​from the perspective of being able to appropriately measure the size of the irregularities of the entire coil.

[0092] In the above derivation process, it is not necessary to determine the size of the irregularities using a two-dimensional coordinate system defined by the protrusion direction of the irregularities on the end surface of the coil and the scanning direction by the displacement gauge. For example, in the above derivation process, the measurement result by the displacement gauge may be derived directly as the size of the irregularities on the end surface of the coil.

[0093] Examples

[0094] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0095] [No.1]

[0096] A hot-rolled steel sheet was manufactured using the manufacturing facility (1) shown in FIG. 2. First, using a non-contact temperature sensor (3a) (a thermograph "CPA-SC7100" from FLIR), the surface temperature of the strip-shaped steel (X) after hot rolling and before being wound onto a coiler (2c) was measured over the entire surface area (full length and full width) of the strip-shaped steel (X) (measurement process). Next, the temperature history in a cooled state of a coil (virtual coil) assuming that the strip-shaped steel (X) was wound into a coil shape without irregularities on the end surface was calculated using the above equations (2) to (5) (first calculation process). In this first calculation process, 10 calculation points were prepared in the central axis direction and 50 in the radial direction (total: 10 × 50), and the temperature history in a cooled state was calculated for each calculation point. Figure 7 shows the temperature history in a cooling state based on the upper surface immediately after winding, which is spaced 410 mm radially from the central axis of the virtual coil above.

[0097] Next, the strip-shaped steel material (X) was actually wound on a winder (2c) (winding process). Then, the end surface (E) of the wound coil (X1) was scanned across the diameter with a displacement meter (5b) (a laser displacement meter "IL-2000" manufactured by KEYENCE) and the shape of the irregularities on the end surface (E) of the coil (X1) was measured. In addition, after setting a reference plane of the end surface (E) based on the median value of the measurement value by the displacement meter (5b), the size of the irregularities on the end surface (E) was derived by the derivation unit (5c) using a two-dimensional coordinate system defined by the direction of the protrusion of the irregularities and the direction of scanning by the displacement meter (5b) (derivation process). In addition, the irregularities on the end surface (E) of the coil (X1) were given a plus if they protruded toward the displacement meter (5b) side and a minus if they were concave toward the conveyor (5a) side. In this derivation process, the coil (X1) is divided into 13 regions in the radial direction, the size of the irregularities in each region is averaged, and this average value is reflected in a two-dimensional coordinate system to derive the size of the irregularities in this region. In this derivation process, to make it easier to reflect irregularities caused by the telescope, the length of a pair of regions located at both ends in the radial direction is set smaller than the length of other regions. Specifically, the length of the regions located at both ends in the radial direction is set to half the length of other regions. In addition, the threshold value of the irregularities is set to 10 mm, and values ​​less than 10 mm are cut off from the average value of the irregularities in each region. FIG. 8 shows the measurement result of the shape of the irregularities on the end surface (E) of the coil (X1) by the displacement meter (5b) in this derivation process and the derivation result of the size of the irregularities on the end surface (E) of the coil (X1) by the derivation unit (5c).

[0098] Next, the temperature history in the cooling state based on the time immediately after winding was predicted by reaching the uneven portion of the end surface (E) of the coil (X1) derived in the extraction process, using the result of the first calculation process and the equations (2) to (5) above (first prediction process). The result of the first prediction process for the end surface (E) of the coil (X1) at a position spaced 410 mm radially from the central axis is shown in FIG. 9. In addition, FIG. 9 shows the actual measured value of the temperature 23 minutes after the time immediately after winding of the end surface (E) of the coil (X1) corresponding to the position predicted in the first prediction process.

[0099] [No.2]

[0100] Using manufacturing equipment (1) similar to No. 1, the measurement process, the first calculation process, the winding process, the extraction process, and the first prediction process were executed in the same manner as No. 1. Figure 10 shows the temperature history in a cooling state based on the time immediately after winding of the upper surface, which is spaced 410 mm radially from the central axis of the virtual coil. In addition, Figure 11 shows the measurement result of the shape of the irregularities on the end surface (E) of the coil (X1) by the displacement gauge (5b) in the extraction process and the extraction result of the size of the irregularities on the end surface (E) of the coil (X1) by the extraction part (5c). In addition, FIG. 12 shows the prediction result of the first prediction process for the end surface (E) of the coil (X1) at a position spaced 410 mm radially from the central axis, and the actual measured value of the temperature 24 minutes after winding the end surface (E) of the coil (X1) corresponding to the position predicted in the first prediction process.

[0101] As shown in FIGS. 7 to 12, together with No. 1, which was determined to have irregularities by the above derivation process, and No. 2, which was determined to have no irregularities, the prediction result by the first prediction process and the actual value are approximately in agreement. Therefore, together with No. 1 and No. 2, it can be seen that the temperature history of the irregularities on the end surface (E) of the coil (X1) is predicted sufficiently accurately.

[0102] [No.3]

[0103] Using a manufacturing device (1) similar to that of No. 1, a measurement process, a first calculation process, a winding process, an extraction process, and a first prediction process were executed. In addition, in No. 3, the phase transformation rate of the end surface of the coil was calculated using the above-described equations (8) and (9), and at the same time, the transformation heat generation amount was calculated using the above-described equation (10) (second calculation process). In No. 3, the temperature history was predicted using the above-described equations (6) and (7) in the first calculation process and the first prediction process. In addition, in No. 3, the Vickers hardness [Hv] at the location (calculation point) where the phase transformation rate was calculated in the second calculation process was measured. The phase transformation rate calculated in No. 3 and the Vickers hardness measured in No. 3 are shown in Table 1.

[0104] Output point Distance from the outer circumference of the coil [mm] Phase transformation rate[-] Vickers Hardness [Hv] A 8.9 0.81 300.4 B 17.7 0.95 243.7 C 26.6 0.98 245.8

[0105] As shown in Table 1, the Vickers hardness at calculation point A, which has a small phase transformation rate, is greater than at calculation points B and C, which have a large phase transformation rate. Therefore, it can be seen that the phase transformation rate is correlated with the hardness of the coil. For this reason, the hardening portion of the coil can be predicted by calculating the phase transformation rate through the second calculation process. In addition, a threshold value for hardness or phase transformation rate at which fracture of the steel may occur in subsequent processes, such as cold rolling, is set in advance, and by comparing this threshold value with the calculated phase transformation rate, it is possible to predict whether fracture of the steel will occur in subsequent processes.

[0106] As explained above, the method for manufacturing a hot-rolled steel sheet according to one aspect of the present invention is suitable for knowing in advance the possibility of fracture of the steel material during the manufacture of a cold-rolled sheet, etc. Explanation of the symbols

[0107] 1: Manufacturing equipment for hot-rolled steel sheets 2: Hot rolling mill 2a: Rolling roll 2b: Return section 2c: Winder 3: Measuring device 3a: Non-contact temperature sensor 4: Output device 5: Extraction device 5a: Conveyor 5b: Displacement gauge 5c: Derivation part 6: Prediction device 7: Cooling device X: Strip-shaped steel X1: Coil X2: Virtual coil E: End face O: Origin R: Reference plane of the coil's end face Z: Center axis of the coil

Claims

Claim 1 A method for manufacturing a hot-rolled steel sheet comprising: a measurement process for measuring the surface temperature of a hot-rolled strip-shaped steel; a first calculation process for calculating the temperature history in a cooling state after winding, assuming that the strip-shaped steel is wound into a coil shape without irregularities on the end surface, based on the surface temperature measured in the measurement process; a winding process for actually winding the strip-shaped steel into a coil shape after the measurement process; a derivation process for scanning the end surface of the coil wound in the winding process with a displacement meter and deriving the size of the irregularities on the end surface over the radius of the coil; and a first prediction process for predicting the temperature history of the irregularities in a cooling state using the temperature history calculated in the first calculation process and the size of the irregularities derived in the derivation process. Claim 2 A method for manufacturing a hot-rolled steel sheet according to claim 1, further comprising a second calculation process for calculating a phase transformation rate using a temperature history predicted in the first prediction process, and a second prediction process for predicting a hardened portion of the strip-shaped steel using the phase transformation rate calculated in the second calculation process, wherein the phase transformation rate refers to a ferrite-pearlite transformation rate. Claim 3 A method for manufacturing a hot-rolled steel sheet according to claim 1 or 2, wherein in the derivation process, the size of the irregularities is determined based on the median value of the measurements taken by the displacement gauge. Claim 4 A method for manufacturing a hot-rolled steel sheet according to claim 1 or 2, wherein in the derivation process, the size of the irregularities is determined using a two-dimensional coordinate system defined by the protrusion direction of the irregularities and the scanning direction by the displacement gauge. Claim 5 A method for predicting the temperature history of a hot-rolled steel sheet, comprising: a measurement process for measuring the surface temperature of a hot-rolled strip-shaped steel sheet; a first calculation process for calculating the temperature history in a cooling state after winding, assuming that the strip-shaped steel sheet is wound into a coil shape without irregularities on the end surface, based on the surface temperature measured in the measurement process; a winding process for actually winding the strip-shaped steel sheet into a coil shape after the measurement process; a derivation process for scanning the end surface of the wound coil wound in the winding process with a displacement meter and deriving the size of the irregularities on the end surface over the radius of the coil; and a first prediction process for predicting the temperature history of the irregularities in a cooling state using the temperature history calculated in the first calculation process and the size of the irregularities derived in the derivation process. Claim 6 A measurement process for measuring the surface temperature of a hot-rolled strip-shaped steel material; a first calculation process for calculating the temperature history in an open-cooling state after winding, assuming that the strip-shaped steel material is wound into a coil shape without irregularities on the end surface, based on the surface temperature measured in the measurement process; a winding process for actually winding the strip-shaped steel material into a coil shape after the measurement process; a derivation process for scanning the end surface of the coil wound in the winding process with a displacement meter and deriving the size of the irregularities on the end surface over the radius of the coil; a first prediction process for predicting the temperature history of the irregularities in an open-cooling state using the temperature history calculated in the first calculation process and the size of the irregularities derived in the derivation process; a second calculation process for calculating the phase transformation rate using the temperature history predicted in the first prediction process; and a second prediction process for predicting the hardened portion of the strip-shaped steel material using the phase transformation rate calculated in the second calculation process. A method for predicting the hardening portion of a hot-rolled steel sheet, comprising a process, wherein the phase transformation rate refers to the ferrite-pearlite transformation rate.

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