Method for controlling steel sheet production facility and steel sheet production facility

Abstract

The present invention provides a technique capable of accurately obtaining, for a steel sheet to be produced, desired mechanical characteristics.　A method for controlling steel sheet production facility includes: a mechanical characteristic prediction step (S3) for predicting mechanical characteristic values including the elongation value and the hole expansion rate of a steel sheet, on the basis of a steel sheet heating temperature in a tempering zone (16) measured by a temperature measurement device (20); a heat treatment condition calculation step (S4) for correcting the mechanical characteristic values predicted in the mechanical characteristic prediction step (S3) to target mechanical characteristic values of the steel sheet (S), and determining a heat treatment condition on the basis of the corrected mechanical characteristic values; and a heat treatment condition control step (S8) for controlling, on the basis of the heat treatment condition determined in the heat treatment condition calculation step (S4), the steel sheet heating temperature in at least a rapid heating zone (9) in continuous annealing equipment.

Description

Control method for steel plate manufacturing equipment and steel plate manufacturing equipment

[0001] The present invention relates to a method for controlling a steel sheet manufacturing facility and a steel sheet manufacturing facility. More specifically, the present invention relates to a method for manufacturing steel sheets used for automotive structural materials, and in particular, to a method for reducing the introduction cost of the manufacturing facility while suppressing the occurrence of material variations in the longitudinal direction of the steel strip.

[0002] In the production of automotive steel sheets, continuously cast slabs undergo extensive hot and cold rolling to reach the final thickness. The subsequent annealing process restores, recrystallizes, and grows the cold-worked structure, controls the transformation structure, and, combined with the cooling process after annealing, adjusts the product's mechanical properties. In recent years, higher-strength steel sheets are required to achieve both lightweight automobiles and crashworthiness. Meanwhile, automotive body structural components are typically manufactured by press working, and products that combine high strength and high formability are required. Furthermore, zinc plating is often used to impart rust resistance to automotive parts. In particular, galvannealed steel sheets, in which zinc and iron are alloyed by heating after plating, are widely used from the perspective of press workability.

[0003] One method for achieving high strength is to utilize a martensite structure obtained by rapidly cooling the austenite phase formed during annealing. However, since the cooled martensite structure is brittle and difficult to handle, toughness can be increased by tempering the material by reheating it. Therefore, Patent Document 1 discloses a method in which the material is rapidly cooled to a martensite transformation temperature or lower in a cooling zone on the outlet side of an annealing furnace, and then reheated and tempered.

[0004] Japanese Patent No. 5402007 Japanese Patent No. 5967318 Japanese Patent Laid-Open No. 2022-024340

[0005] When performing galvanizing and alloying treatment after tempering, as in the method disclosed in Patent Document 1, the alloying temperature is generally higher than the tempering temperature, resulting in excessive tempering due to the alloying heating, and the required strength cannot be obtained. Therefore, Patent Document 2 discloses a method for obtaining the desired coating layer properties and mechanical properties of the steel sheet by performing an alloying treatment in a cooling zone in an annealing furnace while the austenite phase remains, without transforming the steel strip into martensitic, and then performing a tempering process after obtaining a martensitic structure by rapid cooling. However, in actual continuous annealing processes, coils are joined together and subjected to continuous heat treatment, so operating conditions must be changed when the sheet thickness, composition, or target mechanical properties change. If such changes in operating conditions are not reflected in the equipment, portions before and after the joint between coils, where the target mechanical properties differ, will not be manufactured under optimal conditions, resulting in reduced yield. Patent Document 2 does not disclose a method for addressing this issue.

[0006] Regarding such improvement in longitudinal quality stability, Patent Document 3 discloses a technology for controlling the material quality of a steel sheet using a material quality prediction model in which the operational parameters of a continuous annealing facility and transformation rate information of the steel sheet acquired downstream of the continuous annealing facility are used as input data. This technology claims to suppress material quality fluctuations by controlling the operational conditions of the manufacturing equipment downstream of the position where the transformation rate information of the steel sheet is acquired, known as the material quality control zone. However, if there is a control delay in the material quality control zone, the target mechanical properties cannot be obtained at the tip of the steel sheet. However, no solution to this problem has been presented, and therefore the effectiveness is insufficient. As such, the conventional technology has not yet been considered sufficient as a technology for ensuring that the manufactured steel sheet has the desired mechanical properties.

[0007] Therefore, the present invention has been made in consideration of the above problems, and aims to provide a control method for steel plate manufacturing equipment and steel plate manufacturing equipment that make it possible to accurately obtain desired mechanical properties for the steel plate to be manufactured.

[0008] The present inventors have conducted extensive research to solve these problems, and have come to the following conclusion as a result of their intensive investigations. The inventors have conceived the idea that, in a steel sheet manufacturing facility as shown in Fig. 1 , which is the subject of the present invention, the mechanical properties, particularly the elongation value, when a subsequent sheet passes through the facility are predicted based on the heating temperature in the tempering zone of the preceding sheet before and after the joint of the steel strip where the target mechanical properties change, and that by changing the heat treatment conditions in the continuous annealing facility, particularly the steel sheet heating temperature in the rapid heating zone, in real time before the subsequent sheet passes through the facility so as to reduce the difference between the predicted value and the target elongation value, even if there is a delay in changing the heat treatment conditions in the tempering zone when the subsequent sheet passes through the facility, the heat treatment conditions in the rapid heating zone can be changed to match the heat treatment conditions, thereby making it possible to obtain the target mechanical properties over the entire length.

[0009] The present invention has been made based on the above findings and ideas, and has the following features. [1] A method for controlling a steel sheet heating temperature in a continuous annealing facility in a steel sheet manufacturing facility including: a continuous annealing facility for steel sheets having, in this order in a steel sheet transport direction, a heating zone, a soaking zone, a rapid heating zone, and a cooling zone; a post-annealing heat treatment facility having, in this order, at least a rapid cooling zone and a tempering zone downstream of the cooling zone in the steel sheet transport direction; and temperature measuring devices for the steel sheet installed in at least the rapid heating zone and the tempering zone, the method comprising: a mechanical property prediction step of predicting mechanical property values ​​including an elongation value and a hole expansion ratio of the steel sheet based on the steel sheet heating temperature in the tempering zone measured by the temperature measuring device; a heat treatment condition calculation step of correcting the mechanical property values ​​predicted in the mechanical property prediction step to target mechanical property values ​​of the steel sheet and determining heat treatment conditions based on the corrected mechanical property values; and a heat treatment condition control step of controlling the steel sheet heating temperature in at least the rapid heating zone in the continuous annealing facility based on the heat treatment conditions determined in the heat treatment condition calculation step. [2] The control method for steel sheet manufacturing equipment according to [1] above, wherein in the heat treatment condition control step, a steel sheet heating temperature in the rapid heating zone and a steel sheet cooling condition in the cooling zone are controlled based on the heat treatment conditions determined in the heat treatment condition calculation step. [3] The control method for steel sheet manufacturing equipment according to [1] or [2] above, further comprising: a transformation rate measuring device that measures an austenite fraction of the steel sheet at at least one location in the steel sheet transport direction from an outlet side of the soaking zone to an outlet side of the tempering zone, wherein in the mechanical property prediction step, mechanical property values ​​of the steel sheet are predicted based on the steel sheet heating temperature in the tempering zone measured by the temperature measuring device and the austenite fraction measured by the transformation rate measuring device.[4] A steel sheet manufacturing facility comprising: a continuous annealing facility for a steel sheet having, in this order in a steel sheet transport direction, a heating zone, a soaking zone, a rapid heating zone, and a cooling zone; a post-annealing heat treatment facility having, in this order, at least a rapid cooling zone and a tempering zone downstream of the cooling zone in the steel sheet transport direction; and temperature measuring devices for the steel sheet installed at least in the rapid heating zone and the tempering zone, wherein the steel sheet manufacturing facility further comprises: a mechanical property prediction unit that predicts mechanical property values ​​including elongation and hole expandability of the steel sheet based on the steel sheet heating temperature in the tempering zone measured by the temperature measuring device; a heat treatment condition calculation unit that corrects the mechanical property values ​​predicted by the mechanical property prediction unit to target mechanical property values ​​of the steel sheet and determines heat treatment conditions based on the corrected mechanical property values; and a heat treatment condition control unit that controls the steel sheet heating temperature in at least the rapid heating zone in the continuous annealing facility based on the heat treatment conditions determined by the heat treatment condition calculation unit. [5] The steel sheet manufacturing facility according to [4], wherein the heat treatment condition control unit controls the steel sheet heating temperature in the rapid heating zone and the steel sheet cooling conditions in the cooling zone based on the heat treatment conditions determined by the heat treatment condition calculation unit. [6] The steel sheet manufacturing facility according to [4] or [5], further comprising a transformation rate measurement device that measures an austenite fraction of the steel sheet at at least one location in the steel sheet transport direction from an outlet side of the soaking zone to an outlet side of the tempering zone, and wherein the mechanical property prediction unit predicts mechanical property values ​​of the steel sheet based on the steel sheet heating temperature in the tempering zone measured by the temperature measurement device and the austenite fraction measured by the transformation rate measurement device.

[0010] According to the present invention, it is possible to accurately obtain desired mechanical properties for the steel sheet to be manufactured.

[0011] 1 is a diagram for explaining the configuration of a steel sheet manufacturing facility according to the present invention. FIG. 2 is a flow chart for explaining a control method for a steel sheet manufacturing facility according to the present invention.

[0012] <Equipment Configuration> FIG. 1 is a schematic diagram of a steel sheet manufacturing facility 1 according to an embodiment of the present invention. As shown in FIG. 1 , a steel sheet S wound into a coil in a previous process is unwound by a payoff reel 2 and enters a looper 4 along the steel sheet conveying direction (see symbol X in FIG. 1 ). The looper 4 is a facility for ensuring excess length of the steel sheet S so that the steel sheet S can be continuously threaded when the steel sheets S (coils) are joined together with a welder 3. After passing through the looper 4, the steel sheet S enters a continuous annealing facility 5. A heating zone 7 is installed at the entrance side of the continuous annealing facility 5. Alternatively, a preheating zone 6 utilizing combustion exhaust gas generated in the heating zone 7 may be installed before the heating zone 7. The continuous annealing facility 5 is configured with the heating zone 7, followed in this order by a soaking zone 8, a rapid heating zone 9, and a cooling zone 10 (a first cooling zone 10A and a second cooling zone 10B). The cooling zone 10 may be configured with only the first cooling zone 10A in accordance with the cooling amount and strip passing speed required according to the desired mechanical properties, or may be configured with two stages, including the first cooling zone 10A and the second cooling zone 10B, as shown in FIG. 1 .

[0013] Downstream of the cooling zone 10 in the steel sheet transport direction X, a post-annealing heat treatment facility 11 is installed, which includes a rapid cooling zone 15 and a tempering zone 16 in this order. When the steel sheet S to be manufactured is a plated steel sheet (galvannealed steel sheet), the post-annealing heat treatment facility 11 may be a steel sheet plating facility (galvannealed steel sheet), which may include a plating immersion zone (galvannealed steel sheet) 12 and a plating alloying zone 13, in that order, in front of the rapid cooling zone 15 in the steel sheet transport direction X. When the post-annealing heat treatment facility 11 is a steel sheet plating facility (galvannealed steel sheet plating facility), the steel sheet manufacturing facility 1 of the present invention may be a hot-dip galvanized steel sheet manufacturing facility or a hot-dip galvannealed steel sheet manufacturing facility. In this case, the post-annealing heat treatment facility 11 may be provided with a holding zone 14 in order to ensure the time required for the alloying reaction to proceed. The tempering zone 16 is composed of a heating device (tempering zone heating device) 17, a heat retention device (tempering zone heat retention device) 18, and a cooling device (tempering zone cooling device) 19, in this order.

[0014] The steel sheet manufacturing equipment 1 of the present invention is provided with temperature measuring devices 20 for measuring the surface temperatures of the steel sheet at least in the rapid heating zone 9 and the tempering zone 16, so that temperature changes of the steel sheet can be monitored. In addition, a transformation rate measuring device 21 for measuring the austenite fraction of the steel sheet 1 may be provided at least at one location between the exit side of the soaking zone 8 and the exit side of the tempering zone 16.

[0015] The steel sheet manufacturing equipment 1 of the present invention may also include a management device 30. The management device 30 includes a mechanical property prediction unit 31 that predicts mechanical property values, including elongation and hole expansion ratio, of the steel sheet S based on the steel sheet heating temperature in the tempering zone 16 measured by the temperature measurement device 20; a heat treatment condition calculation unit 32 that corrects the mechanical property values ​​predicted by the mechanical property prediction unit 31 to target mechanical property values ​​(e.g., target elongation, target hole expansion ratio) of the steel sheet S and determines heat treatment conditions based on the corrected mechanical property values; and a heat treatment condition control unit 33 that controls the steel sheet heating temperature in at least the rapid heating zone 9 of the various equipment constituting the continuous annealing equipment 5 based on the heat treatment conditions determined by the heat treatment condition calculation unit 32. The steel sheet manufacturing equipment 1 of the present invention includes the mechanical property prediction unit 31, the heat treatment condition calculation unit 32, and the heat treatment condition control unit 33, thereby suppressing deviations in the mechanical properties of the resulting steel sheet from the desired mechanical properties. Details of the functions of these devices will be described later with reference to FIG. 2.

[0016] <Details of Each Facility> Details of each facility included in the steel sheet manufacturing facility 1 of the present invention will be described.

[0017] (1) Preheating Zone 6 The steel sheet S discharged from the payoff reel 2 at a temperature of about room temperature to 100°C passes through the looper 4 and enters the preheating zone 6. The preheating zone 6 is used to improve energy efficiency in the heating zone 7 and soaking zone 8, which will be described later. However, even if the steel sheet manufacturing equipment 1 has the preheating zone 6, heating of the steel sheet S, such as a thin steel sheet, may begin in the next heating zone 7 in the steel sheet conveying direction X without heating in the preheating zone 6. When the steel sheet S is heated in the preheating zone 6, the steel sheet S, such as a thin steel sheet, is heated to about 200°C. A possible heating method for the preheating zone 6 is to utilize the high-temperature exhaust air generated in the next heating zone 7, but there are no particular limitations as long as the target temperature is reached.

[0018] (2) Heating Zone 7 Next, the steel sheet S enters the heating zone 7, where the steel sheet temperature is heated to a heating temperature of 600°C or higher, which is similar to the annealing temperature (soaking temperature), in order to maximize the holding time in the soaking zone. This heating temperature may be 700°C or lower. In the heating zone 7, a direct-fire heating furnace is preferably used because it has a high heating capacity, allowing the furnace volume to be small, and, when plating the steel sheet S, it is possible to flexibly control the oxidation-reduction reaction on the steel sheet surface to ensure plating properties in subsequent processes. This method makes it possible to heat the steel sheet to the desired temperature in a short time, making it easy to control the condition of the steel sheet surface.

[0019] (3) Soaking Zone 8 In the subsequent soaking zone 8, the steel sheet heated in the heating zone 7 is held or slowly heated. The heating method in the soaking zone 8 is preferably a radiant heating method using gas combustion (radiant tube heating) due to its high efficiency and heating uniformity, but is not particularly limited thereto. In the present invention, the final annealing temperature is determined in the subsequent rapid heating zone 9. If the fine-grained α phase is heated to the α / γ two-phase region in a short time in the subsequent rapid heating zone 9, the transformation from α phase to γ ​​phase will proceed rapidly, making it difficult to obtain the target microstructure fraction. Therefore, in the soaking zone 8, slow heating or holding is performed at a temperature above the A1 transformation point to promote recrystallization of the α phase. Holding or slow heating in a temperature range below the A1 transformation point ensures a residence time in the recrystallization temperature range and suppresses the retention of unrecrystallized α phase. In this case, if the residence time in the recrystallization temperature range is too short, the recrystallization of the α phase will not progress sufficiently, but if it is too long, the crystal grains will become coarse and the mechanical properties will deteriorate. Therefore, the residence time in this temperature range is preferably 20 seconds or more. Furthermore, the residence time in this temperature range is preferably 60 seconds or less. Even when slow heating is performed in the soaking zone 8, the residence time in the recrystallization temperature range is preferably secured for the above-mentioned time (20 seconds or more, or even 60 seconds or less). After the recrystallization time is secured, the steel may be slowly heated to a temperature equal to or higher than the A1 transformation point, as long as it is equal to or lower than the target annealing temperature described below. However, the heating rate during slow heating in the recrystallization temperature range is preferably 5°C / s or less. This is because there is a concern that heating at a rate exceeding 5°C / s may make it impossible to suppress the retention of the unrecrystallized α phase.

[0020] (4) Rapid Heating Zone 9 After the α-phase recrystallization has progressed in the soaking zone 8, the steel sheet S must be heated in the rapid heating zone 9 to a target annealing temperature set in a temperature range higher than the A1 transformation point and lower than the A3 transformation point of the steel sheet S. Because the temperature reached at this time has a significant impact on the final mechanical properties, it is preferable to select an induction heating method, which has a fast response to temperature control. Furthermore, since the steel sheet S is heated to a high temperature above the A1 transformation point, it exceeds the Curie point at which the magnetic properties of the steel sheet S change, so a transverse induction heating device is preferable. Because the steel sheet S is maintained in the α-phase recrystallization region in the soaking zone 8, excessive heating time can lead to coarsening of the α-grains. Therefore, a heating rate of 10°C / s or higher is preferable. Furthermore, it is desirable to reduce the equipment length and construction costs. Considering such equipment costs, a heating rate of 20°C / s or higher is more preferable. On the other hand, if the heating rate is increased to increase the temperature rise, localized high-temperature areas will occur in the width direction, impairing the uniformity of the mechanical properties of the entire steel sheet. Therefore, a heating rate of 200°C / s or lower is preferable. However, when a thin steel sheet is rapidly heated, thermal stress can cause the steel sheet to buckle and deform, potentially resulting in transport problems. Therefore, considering operational stability, a heating rate of 100°C / s or less is preferable. Rapid heating to the target annealing temperature using an induction heating device or the like can result in the transformation from the α phase to the γ phase not reaching equilibrium immediately after heating. In this case, holding the steel sheet near the target annealing temperature can further accelerate the transformation from the α phase to the γ phase, even if the steel sheet is isothermally held. Because the holding time varies depending on the line speed, there is a concern that holding the steel sheet near the target annealing temperature can complicate material control. Therefore, after reaching the target annealing temperature, it is desirable to enter the cooling zone 10 as quickly as possible, with cooling preferably commencing within 10 seconds, and more preferably within 5 seconds. The rapid heating device, such as an induction heating device, used in the rapid heating zone 9 is preferably installed immediately before the cooling zone 10, regardless of the structure of the soaking zone 8. If the rapid heating device is added to an existing furnace, it can be installed at the connection between the soaking zone 8 and the cooling zone 10.Furthermore, if the soaking zone 8 is divided into multiple sections, multiple rapid heating devices may be installed in a location other than immediately before the cooling zone 10. For example, the soaking zone 8 may be divided into two sections, with the upstream section maintaining the steel sheet in the recrystallization temperature range and the downstream section maintaining the steel sheet in the high temperature range to promote the reduction reaction on the steel sheet surface. In such a case, the optimal temperature range for each section will differ. Therefore, a first rapid heating device installed after the upstream soaking zone 8 heats the steel sheet from the low temperature section to the high temperature section, and a second rapid heating device is installed after the downstream soaking zone 8 and immediately before the steel sheet enters the cooling zone 10 for the purpose of temperature adjustment for material quality control.

[0021] (5) Cooling Zone 10 (First Cooling Zone 10A, Second Cooling Zone 10B) After being heated to the target annealing temperature in the rapid heating zone 9, the steel sheet S is transported to the cooling zone 10 and cooled. It is preferable to set a cooling rate to prevent ferrite transformation during cooling due to an insufficient cooling rate and to prevent rapid bainite transformation after cooling is stopped due to an excessive cooling rate. Specifically, the cooling rate range is preferably 5°C / s or more. Furthermore, the cooling rate range is preferably 30°C / s or less. In order to prevent martensitic transformation from occurring upstream of the plating process and to transform a portion of the austenite phase into the bainite phase, the cooling stop temperature is preferably set to a martensitic transformation start temperature or higher and 550°C or lower, more preferably in the range of 450°C or higher and 550°C or lower. Furthermore, in order to sufficiently promote bainite transformation, it is preferable to hold the steel sheet S at the cooling stop temperature for 1 second or longer, and more preferably for 20 seconds or longer, after reaching the cooling stop temperature. The holding time is preferably 100 seconds or less, and more preferably 50 seconds or less. Possible cooling methods include gas jet cooling, which involves the collision of a compressed gas jet with the material; roll cooling, which involves cooling by contact with a roll through which a refrigerant is passed; water cooling using a water jet; and mist cooling, which involves a mixture of compressed gas and minute water droplets. In the present embodiment described below, gas jet cooling was employed, which can accurately control the cooling stop temperature while ensuring the target cooling rate and preventing unstable temperature changes due to boiling. Furthermore, as shown in FIG. 1 , if slow cooling or heat retention is performed after cooling to further promote the bainite transformation, the cooling zone 10 may be divided into two zones: a first cooling zone (rapid cooling zone) 10A and a second cooling zone (slow cooling / holding zone) 10B.

[0022] (6) Galvanizing Zone (Hot-Dip Galvanizing Zone) 12, Galvanizing Alloying Zone 13, and Holding Zone 14 After microstructure control in the cooling zone 10, the steel sheet S is transported to a post-annealing heat treatment facility 11. When a galvanizing treatment is performed on the steel sheet S, the post-annealing heat treatment facility 11 may be a steel sheet plating facility, and the steel sheet S is transported to this steel sheet plating facility and subjected to a galvanizing treatment (e.g., galvanizing treatment) in the galvanizing zone 12. In the present invention, the galvanizing method may be a hot-dip galvanizing method in which the steel sheet is immersed in a galvanizing bath containing molten zinc. Note that the post-annealing heat treatment facility (steel sheet plating facility) 11 may be equipped with a device for scraping off excess molten zinc in order to control the amount of zinc deposited after passing through the galvanizing bath. A galvanizing alloying zone 13 for promoting the Zn—Fe alloying reaction may be provided downstream of the galvanizing zone 12. At this time, the steel sheet temperature has dropped to about 430°C, so it is heated in the plating alloying zone 13 to the temperature required for the alloying reaction (about 500°C). Possible heating methods in the plating alloying zone 13 include gas heating using combustion exhaust gas, induction heating, and electric heating, but are not particularly limited. In the present invention, an induction heating method may be used, which has little effect on the plating surface and allows for precise output control. A holding zone 14 may be installed downstream of the plating alloying zone 13 to ensure the time required for the alloying reaction to proceed.

[0023] (7) Rapid Cooling Zone 15 When the steel sheet S is subjected to a plating treatment (when the post-annealing heat treatment equipment 11 is used as a steel sheet plating equipment), a rapid cooling zone 15 is installed following the plating alloying zone 13 or the holding zone 14, in order to transform the untransformed austenite phase into a martensite structure after the plating alloying reaction is completed. When the steel sheet S is not subjected to a plating treatment, the rapid cooling zone 15 may be installed downstream of the cooling zone 10 in the steel sheet conveying direction X. Possible cooling methods in the rapid cooling zone 15 include, but are not limited to, gas jet cooling in which a compressed gas jet is impinged, water cooling using a water jet, and mist cooling in which compressed gas and fine water droplets are mixed. In the present invention, a combination of mist cooling and gas jet cooling may be used to achieve rapid cooling while controlling the cooling stop temperature with high precision. When a steel sheet S is subjected to a plating treatment, the steel sheet S passes through a plating alloying zone 13 or a holding zone 14, and is cooled to approximately 350 to 450°C. The steel sheet S is then cooled in a rapid cooling zone 15 to a temperature below the martensitic transformation start temperature. To obtain a desired martensite structure while preventing excessive bainite transformation, the cooling rate is preferably 50°C / s or higher. While there is no particular upper limit to the cooling rate, excessively rapid cooling may result in loss of flatness of the steel sheet due to thermal deformation during cooling. Therefore, the cooling rate is preferably 1000°C / s or lower. Furthermore, the cooling stop temperature is preferably 200°C or lower to prevent self-tempering after cooling.

[0024] (8) Tempering Zone 16 After the martensitic structure is created in the rapid cooling zone 15, tempering is performed in the tempering zone 16. The tempering zone 16 is configured, in this order from the inlet in the steel sheet conveying direction X, with a heating device (tempering zone heating device) 17, a heat retention device (tempering zone heat retention device) 18, and a cooling device (tempering zone cooling device) 19. The heating temperature in the heating device 17 may be set from the viewpoint of obtaining the effect of improving toughness by tempering while maintaining the martensitic structure. Specifically, the heating temperature in the heating device 17 is preferably 250°C or higher, more preferably 300°C or higher. The heating temperature in the heating device 17 is preferably 500°C or lower, more preferably 400°C or lower. The heat retention device 18 maintains the temperature of the steel sheet heated in the heating device 17, and the holding time in the heat retention device 18 may be set from the viewpoint of obtaining a sufficient tempering effect while preventing excessive extension of the line length. Specifically, the holding time in the heat retention device 18 is preferably 20 seconds or more, more preferably 30 seconds or more. Furthermore, the holding time in the heat retention device 18 is preferably 100 seconds or less, more preferably 60 seconds or less. The steel sheet S is then cooled to room temperature in the cooling device 19. This configuration allows for appropriate tempering treatment and the target mechanical properties to be obtained. Heating in the heating device 17 can be performed using a gas burner, electrical heating, induction heating, or other methods. In the present invention, an induction heating method may be used from the perspective of controllability. From the perspective of temperature uniformity within the device, the heat retention device 18 preferably uses a radiant heating method from the heater body and furnace wall using an electric heater, but this is not particularly limited. In a steady state, the furnace temperature may be controlled to be approximately the same as the target tempering temperature. From the perspective of cooling efficiency, the cooling device 19 is preferably a mist cooling method, and a gas jet device is preferably installed to remove moisture adhering to the surface simultaneously with cooling. However, this is not particularly limited as long as the target cooling rate can be adjusted.

[0025] (9) Temperature Measuring Device 20 Of the above-mentioned zones, at least the rapid heating zone 9 and the tempering zone 16 are equipped with temperature measuring devices (thermometers) 20 for measuring the surface temperature of the steel sheet. These temperature measuring devices 20 make it possible to know the temperature history of the steel sheet S during heat treatment. When a thermometer is installed in a long facility such as the soaking zone 8, a thermometer may also be installed within the zone to check the temperature history during the treatment. Specific installation positions of the thermometers within each zone include, for example, the soaking zone entrance (= heating zone exit), the middle of the soaking zone, and the soaking zone exit (= rapid heating zone entrance) in the soaking zone 8. Furthermore, examples of installation positions of the thermometers within each zone include the rapid heating zone entrance (= soaking zone exit) and the rapid heating zone exit (= cooling zone entrance) in the rapid heating zone 9, and the tempering zone entrance, middle of the tempering zone, and the tempering zone exit in the tempering zone 16. The temperature measurement method used by the temperature measuring device 20 is not particularly limited, but it is preferable to use a radiation thermometer that measures the temperature by detecting infrared rays emitted by the steel sheet. Since the radiation thermometer is affected by the reflected light of infrared rays emitted by the surrounding furnace body, a cover may be provided between the measurement unit and the detection unit of the radiation thermometer. Furthermore, since the radiation thermometer is also affected by the emissivity of the steel sheet surface, a multiple reflection measurement method that utilizes the wedge-shaped space between the in-furnace transport roll and the steel sheet S may be adopted as the temperature measurement method.

[0026] (10) Transformation Ratio Measuring Device 21 It is preferable that one or more transformation ratio measuring devices 21 for measuring the austenite fraction of the steel sheet S are installed between the outlet of the soaking zone 8 and the outlet of the tempering zone 16. This is because predicting the mechanical property values ​​(e.g., elongation, hole expansion ratio) of the steel sheet S based on the measured austenite fraction improves the prediction accuracy of the mechanical property values ​​of the steel sheet S compared to predicting the mechanical property values ​​such as the elongation and hole expansion ratio of the steel sheet S from only the steel sheet heating temperature in the tempering zone 16. This transformation ratio measuring device 21 can obtain information on the fractions (transformation ratios) of the α and γ phases of the steel sheet after the target annealing temperature is reached. This transformation ratio measuring device 21 is provided for the purpose of adjusting the operating conditions of the continuous annealing equipment 5 based on the measured austenite fraction, thereby enabling desired mechanical properties to be stably obtained. The method for measuring the transformation rate is not particularly limited, but the transformation rate measuring device 21 may be a magnetic detector, i.e., a device for measuring the magnetic transformation rate of the steel sheet S (steel strip), and the austenite fraction can be measured as a magnetic transformation rate measuring device composed of a drive coil that generates a magnetic field and a detection coil that measures the magnetic field that has passed through the steel sheet S. As another technique, for example, a method applying X-ray diffraction may be employed. In this method applying X-ray diffraction, the γ phase and the α phase each produce diffraction peaks at specific angles when X-rays are irradiated onto the steel sheet due to differences in their crystal structures, and the austenite fraction can be quantified from the intensity of these diffraction peaks.

[0027] (Mechanical Property Values ​​(Elongation Value, Hole Expansion Ratio)) As a mechanical property value of the steel sheet S of the present invention, the elongation value is determined, for example, based on the test method of JIS Z2241 (2011). The sample for measuring the elongation value is preferably taken from the steel sheet that has been shear-cut after treatment in the tempering zone cooling device 19 of the present invention and immediately before coiling. For each coil, samples are taken from the leading and trailing ends of the coil during operation. As a mechanical property value of the steel sheet S of the present invention, the hole expansion ratio is determined, for example, based on the test method of JIS Z2256 (2010). The sample for measuring the hole expansion ratio is preferably taken from the steel sheet that has been shear-cut immediately before coiling. For each coil, samples are taken from the leading and trailing ends of the coil during operation.

[0028] <Control Method> Fig. 2 is a flow chart for explaining a control method for steel sheet manufacturing equipment in the present invention. The configurations of the mechanical property prediction unit 31, heat treatment condition calculation unit 32, and heat treatment condition control unit 33 included in the management device 30 and the processes performed by these will be described with reference to the flowchart in Fig. 2.

[0029] (1) Mechanical Property Prediction Unit 31, Mechanical Property Prediction Step In the steel sheet manufacturing equipment 1, a new steel sheet S is manufactured (step S1), and it is determined whether the coil requires a change in manufacturing conditions at the joint portion of the steel sheet S (coil), i.e., at the connection position between the preceding material and the succeeding material (step S2). If a change in manufacturing conditions is not required, the steel sheet manufacturing equipment 1 continues manufacturing the steel sheet S without changing the manufacturing conditions. This determination in step S2 may be made by a control unit (not shown) in the management device 30 of the steel sheet manufacturing equipment 1, or by another control unit (not shown) possessed by the steel sheet manufacturing equipment 1. For example, if the plate thickness, steel composition, and target mechanical properties differ between the preceding material and the succeeding material, the management device 30 determines that a change in manufacturing conditions is required. If a change in manufacturing conditions is required, in mechanical property prediction step S3, the mechanical property prediction unit 31 predicts the mechanical property values ​​of the steel sheet S (successing material) based on the operating conditions (steel sheet heating temperature) in the tempering zone (reheating zone) 16. Examples of mechanical property values ​​include elongation and hole expansion ratio. Other operational conditions input into the mechanical property prediction unit 31 include the operational conditions of the equipment downstream of the rapid heating zone 9. The mechanical properties are predicted under these operational conditions when the subsequent material completes the process up to the tempering zone (reheating zone) 16. In this case, the austenite fraction measured by the transformation rate measurement device 21 is further added to the operational conditions, and the mechanical property values ​​are predicted based on this austenite fraction and the operational conditions, thereby improving the accuracy of the prediction of the mechanical property values. Because temperature measurement alone results in an indirect prediction of the structure, directly measuring the austenite fraction also improves the accuracy of the prediction of the mechanical properties after the heat treatment process. Regarding the elongation and hole expansion ratio, the mechanical property prediction unit 31 may use, for example, an empirical formula calculated by regression analysis based on the results of heat treatment experiments, or an elongation prediction database constructed using a data science method based on the operational conditions and material measurement results. However, the prediction method is not particularly limited. In the present invention, from the viewpoint of improving the prediction accuracy for new product types, a regression analysis method based on the experimental results of the heat treatment of each steel sheet can be used.In this procedure, heat treatment simulating an actual machine is performed offline or experimentally, and in the case of elongation value, regression analysis is performed based on the results of the test method of JIS Z2241 (2011), and in the case of hole expansion ratio, regression analysis is performed based on the results of the test method of JIS Z2256 (2010), to predict the elongation value and hole expansion ratio. The mechanical property prediction unit 31 can record data in the memory unit 34 having the above-mentioned elongation value prediction database, and can predict the elongation value, etc. of the steel sheet S based on the data.

[0030] (2) Heat Treatment Condition Calculation Unit 32, Heat Treatment Condition Calculation Step In the heat treatment condition calculation step S4, the mechanical property values ​​of the steel sheet S (subsequent material) predicted by the mechanical property prediction unit 31 in the mechanical property prediction step S3 are corrected to target mechanical property values, and the heat treatment conditions in the continuous annealing equipment 5 are calculated based on the corrected mechanical property values. The term "correction" used here refers to changing the mechanical property values ​​used to calculate the heat treatment conditions from the predicted mechanical property values ​​to preset target mechanical property values. If the heat treatment condition calculation unit 32 determines that the target mechanical property values ​​can be corrected by simply changing the steel sheet heating conditions in the rapid heating zone 9 (step S5), it calculates and determines the steel sheet heating temperature conditions in the rapid heating zone 9 (step S6a). On the other hand, if the heat treatment condition calculation unit 32 determines that the target mechanical property values ​​cannot be corrected by simply changing the steel sheet heating temperature in the rapid heating zone 9 (step S5), it calculates the steel sheet cooling conditions in the cooling zone 10 in addition to the heating temperature in the rapid heating zone 9 (step S6b). Methods for determining the heat treatment conditions (the steel sheet heating temperature in the rapid heating zone 9 and the steel sheet cooling conditions in the cooling zone 10) include, but are not limited to, an empirical formula calculated by performing regression analysis based on experimental results, or a heat treatment condition database constructed using a data science approach based on operational conditions and material measurement results. In the present invention, from the perspective of prediction accuracy for new product types, a regression analysis method based on the experimental results of the heat treatment of each steel sheet can be used. This procedure involves performing heat treatment simulating an actual machine offline or experimentally, and then performing regression analysis based on the results to predict elongation, hole expansion ratio, and the like. The steel sheet cooling conditions in the cooling zone 10 include, but are not limited to, the cooling stop temperature and cooling rate in the cooling zone 10. These cooling stop temperature and cooling rate can be obtained by measuring them with a thermometer installed in the cooling zone 10. The heat treatment condition calculation unit 32 can record data in a memory unit 34 having the above-mentioned heat treatment condition database, and can calculate and determine the heat treatment conditions for the steel sheet S based on the data.

[0031] (3) Heat Treatment Condition Control Unit 33, Heat Treatment Condition Control Step In the heat treatment condition control step S7, the heat treatment condition control unit 33 controls the steel sheet heating temperature in the rapid heating zone 9 or the steel sheet cooling conditions in the cooling zone 10 based on the heat treatment conditions determined by the heat treatment condition calculation unit 32. By controlling not only the steel sheet heating temperature in the rapid heating zone 9 but also the steel sheet cooling conditions in the cooling zone 10 based on the above heat treatment conditions, the mechanical properties (elongation value, hole expansion ratio) of the steel sheet S (following zone) can be controlled with higher precision.

[0032] (4) Transition to Steady State The management device 30 determines whether the heat treatment conditions determined based on the steel sheet heating temperature measured in the tempering zone (reheating zone) 16 satisfy the target heat treatment conditions for the subsequent sheet (step S8). If it is determined that the target heat treatment conditions for the subsequent sheet are satisfied, the management device 30 stops the control in the mechanical property prediction step S3, heat treatment condition calculation step S4, and heat treatment condition control step S7, and adjusts the heat treatment conditions (steady state heat treatment conditions) other than the target heat treatment conditions for the subsequent sheet (transition to steady state). Here, steady state refers to a state in which the heat treatment conditions, including the target heat treatment conditions, have reached the desired conditions. This ends the control process for the steel sheet manufacturing equipment in the present invention. On the other hand, if it is determined that the target heat treatment conditions for the subsequent sheet are not satisfied, mechanical property prediction is performed again (step S3).

[0033] 2 , for example, even if there is a concern that a control delay will occur and the operating conditions in the tempering zone 16 will differ from the target tempering conditions for the subsequent material, preventing the desired mechanical property values ​​from being obtained, the control delay in the tempering zone 16 can be canceled out by inputting information about the steel sheet heating temperature to the mechanical property prediction unit 31, calculating the heat treatment conditions in the continuous annealing equipment 5 required for correction to the target mechanical property values ​​in the heat treatment condition calculation unit 32, and controlling the heat treatment conditions in the rapid cooling zone 9 and / or the cooling zone 10 with the heat treatment condition control unit 33. In this way, it is possible to reduce the deviation of the mechanical properties of the resulting steel sheet from the desired mechanical properties, and it becomes possible to stably obtain the target mechanical properties along the longitudinal direction, as compared to conventional steel sheet manufacturing equipment.

[0034] Thin steel sheets (coils, steel strips) were produced using the steel sheet manufacturing equipment according to the embodiment of the present invention described above. To investigate the variation in the mechanical properties and yield of the products, three types of products with strength levels of 780 MPa, 980 MPa, and 1180 MPa were produced in succession. Five slabs of each strength were threaded in succession, and a total of 15 slabs of each type were produced in ten sets, totaling 150 slabs for each example. The thickness ranged from 1.0 to 2.0 mm, and the production order was determined so that the slabs within each set had a nearly constant thickness. Sets with gradually varying thicknesses were connected to produce the products. Furthermore, products of the same strength within the same set were produced using slabs cast in different lots in a continuous casting machine. More specifically, although within the production control range, the chemical compositions of each slab varied, and the transformation behavior was not uniform.

[0035] Each slab was hot-rolled, pickled, and annealed and cold-rolled as needed, followed by heat treatment using conventional annealing equipment or the annealing equipment of the present invention. The annealing equipment was then cooled and subjected to other post-treatments, such as plating. Measurement results of samples taken 10 m from the end of the coil of the final product were used as representative values ​​to examine the variation in properties between products of the same strength. Furthermore, at the joints between products of different strengths, analytical samples were taken every 10 m within a range of 100 m both before and after the joint. Areas where the mechanical properties did not meet the shipping standards were examined, and the ratio of the shippable length to the original coil length was calculated as the joint yield. A joint yield of 90% or higher was considered acceptable. The tensile test specimens were JIS No. 5, and the tensile tests were conducted in accordance with JIS Z2241 (2011). For the 780 MPa, 980 MPa, and 1180 MPa grades, the required strength ranges are 780 MPa or more, 980 MPa or more, and 1180 MPa or more, respectively, and the ductility is 17% or more, 15% or more, and 12% or more. The hole expansion test was performed in accordance with JIS Z2256 (2010). The required hole expansion ratios for the 780 MPa, 980 MPa, and 1180 MPa grades are 65% or more, 50% or more, and 40% or more.

[0036] The operating conditions of the annealing furnace were controlled so that the steel plate temperature fell within the specified range for each product. The line speed was within the range of 60 to 120 mpm, and the change in steel plate temperature due to plate thickness was controlled. The steel plate temperature at the exit of the heating zone (direct flame heating) was within the range of 600 to 700°C, and the annealing temperature was within the range of 750 to 870°C. The furnace temperature in the soaking zone was controlled so that the steel plate temperature at the exit reached the target value. After cooling, hot-dip galvanizing, alloying, etc., the final product was subjected to treatment, and multiple material test pieces were taken from the product to investigate the variation in mechanical properties.

[0037] Table 1 shows the results of the production under each production condition.

[0038]

[0039] Comparative Example 1 is an example in which a product was manufactured from the above-mentioned material using a conventional CGL comprising a preheating zone, a heating zone, a soaking zone, a cooling zone, a plating zone, a plating alloying zone, and a final cooling zone, as described in Cited Document 1. Comparative Example 2 is an example in which a tempering zone was provided after the plating alloying zone, as compared to Comparative Example 1. Comparative Example 3 is an example in which a rapid heating zone having an induction heating device was provided at the outlet of the soaking zone in Comparative Example 2. Comparative Example 4 is an example in which a transformation rate measuring device was installed downstream of the induction heating device in Comparative Example 3. In these comparative examples, even when a tempering zone (reheating zone) was used, control of the continuous annealing equipment based on the steel sheet heating temperature in the tempering zone was deemed "not implemented."

[0040] As shown in Table 1, in Comparative Example 1, the TS varied greatly within products of the same strength, with some falling below the lower limit for each grade. Furthermore, because tempering was not performed in the tempering zone, it was not possible to manufacture products that met the ductility standard, and the yield at the joint could not be calculated. In Table 1, the yield is indicated by "-".

[0041] In Comparative Example 2, tempering in the tempering zone improved ductility compared to Comparative Example 1, making it possible to manufacture products that met the acceptance criteria, but delays in controlling the heating temperature in the annealing furnace and the tempering zone resulted in many areas where the steel sheet temperature control was off target. Furthermore, there was a large variation in mechanical properties within the coil, and wide areas occurred in the joints that did not meet the material testing criteria, resulting in low yields at the joints.

[0042] In Comparative Example 3, the control range of the annealing temperature was expanded by the induction heating device, and no TS deviation occurred. However, since the continuous annealing equipment was not controlled based on the steel sheet heating temperature in the tempering zone, the ductility varied greatly, and some steel sheets did not meet the acceptance criteria. In addition, the yield at the joint was not sufficiently improved.

[0043] In Comparative Example 4, the use of a transformation rate measuring device improved the controllability of the annealing temperature by the induction heating device, and the TS variation was further improved. However, since the continuous annealing equipment was not controlled based on the steel sheet heating temperature in the tempering zone, the ductility variation was not improved. Furthermore, the yield at the joint was not improved.

[0044] In contrast to these comparative examples, Example 1 shows the results of manufacturing a product from the above-mentioned material using the method for controlling steel sheet manufacturing equipment according to the present invention. Example 3 is an example in which a transformation rate measuring device was installed downstream of the induction heating device of Example 1. In Examples 1, 2, and 3, control of the continuous annealing equipment based on the steel sheet heating temperature in the tempering zone was "implemented." As a result, by controlling the continuous annealing equipment based on the steel sheet heating temperature in the tempering zone at the joint of the coil, it was determined that almost all products at the joint, except for those near the weld, were shippable. Furthermore, controlling the annealing temperature based on the austenite fraction measured by the transformation rate measuring device further improved the TS variation.

[0045] S: Steel sheet X: Steel sheet transport direction 1: Steel sheet manufacturing equipment 2: Payoff reel 3: Welder 4: Looper 5: Continuous annealing equipment 6: Preparing zone 7: Heating zone 8: Soaking zone 9: Rapid heating zone 10: Cooling zone 10A: First cooling zone 10B: Second cooling zone 11: Post-annealing heat treatment equipment (steel sheet plating equipment) 12: Plating immersion zone 13: Plating alloying zone 14: Holding zone 15: Rapid cooling zone 16: Tempering zone 17: Tempering zone heating device 18: Tempering zone heat retention device 19: Tempering zone cooling device 20: Temperature measuring device 21: Transformation rate measuring device 30: Management device 31: Mechanical property prediction unit 32: Heat treatment condition calculation unit 33: Heat treatment condition control unit 34: Memory unit

Claims

1. A method for controlling a steel sheet heating temperature in a continuous annealing facility in a steel sheet manufacturing facility, the method comprising: a continuous annealing facility for steel sheets having, in a steel sheet transport direction, a heating zone, a soaking zone, a rapid heating zone, and a cooling zone, in this order; a post-annealing heat treatment facility having, downstream of the cooling zone in the steel sheet transport direction, at least a rapid cooling zone and a tempering zone, in this order; and temperature measuring devices for the steel sheet, which are installed at least in the rapid heating zone and the tempering zone. The method includes a mechanical property prediction step of predicting mechanical property values ​​including an elongation value and a hole expansion ratio of the steel sheet based on the steel sheet heating temperature in the tempering zone measured by the temperature measuring device; and a heat treatment condition calculation step of correcting the mechanical property values ​​predicted in the mechanical property prediction step to target mechanical property values ​​of the steel sheet, and determining heat treatment conditions based on the corrected mechanical property values. a heat treatment condition control step of controlling a steel sheet heating temperature in at least the rapid heating zone in the continuous annealing facility based on the heat treatment conditions determined in the heat treatment condition calculation step.

2. A method for controlling steel plate manufacturing equipment as described in claim 1, wherein in the heat treatment condition control step, the steel plate heating temperature in the rapid heating zone and the steel plate cooling conditions in the cooling zone are controlled based on the heat treatment conditions determined in the heat treatment condition calculation step.

3. A control method for steel plate manufacturing equipment as described in claim 1 or 2, further comprising a transformation rate measuring device in the steel plate manufacturing equipment that measures the austenite fraction of the steel plate at at least one location in the steel plate transport direction from the soaking zone exit side to the tempering zone exit side, and the mechanical property prediction step predicts a mechanical property value of the steel plate based on the steel plate heating temperature in the tempering zone measured by the temperature measuring device and the austenite fraction measured by the transformation rate measuring device.

4. A steel sheet manufacturing facility comprising: a continuous annealing facility for steel sheet having a heating zone, a soaking zone, a rapid heating zone, and a cooling zone, in this order, in a steel sheet transport direction; a post-annealing heat treatment facility having at least a rapid cooling zone and a tempering zone, in this order, downstream of the cooling zone in the steel sheet transport direction; and temperature measuring devices for the steel sheet installed at least in the rapid heating zone and the tempering zone, the steel sheet manufacturing facility further comprising: a mechanical property prediction unit that predicts mechanical property values ​​of the steel sheet, including an elongation value and a hole expansion ratio, based on the steel sheet heating temperature in the tempering zone measured by the temperature measuring device; a heat treatment condition calculation unit that corrects the mechanical property values ​​predicted by the mechanical property prediction unit to target mechanical property values ​​of the steel sheet and determines heat treatment conditions based on the corrected mechanical property values; and a heat treatment condition control unit that controls the steel sheet heating temperature in at least the rapid heating zone in the continuous annealing facility, based on the heat treatment conditions determined by the heat treatment condition calculation unit.

5. A steel plate manufacturing equipment as described in claim 4, wherein the heat treatment condition control unit controls the steel plate heating temperature in the rapid heating zone and the steel plate cooling conditions in the cooling zone based on the heat treatment conditions determined by the heat treatment condition calculation unit.

6. A steel plate manufacturing facility as described in claim 4 or 5, further comprising a transformation rate measuring device that measures the austenite fraction of the steel plate at at least one location in the steel plate transport direction from the soaking zone exit side to the tempering zone exit side, wherein the mechanical property prediction unit predicts mechanical property values ​​of the steel plate based on the steel plate heating temperature in the tempering zone measured by the temperature measuring device and the austenite fraction measured by the transformation rate measuring device.

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