Control method of steel plate manufacturing facility and steel plate manufacturing facility

Abstract

To provide a technique capable of accurately obtaining desired mechanical characteristics, for a steel plate to be manufactured.SOLUTION: A control method of a steel plate manufacturing facility includes: a mechanical characteristic prediction step S3 of predicting a mechanical characteristic value including an elongation value of a steel plate and a hole expansion rate, on the basis of a steel plate heating temperature in a tempering strip 16 measured by a temperature measuring device 20; a heat treatment condition calculation step S4 of correcting the mechanical characteristic value predicted by the mechanical characteristic prediction step S3 to a target mechanical characteristic value of a steel strip S, and determining a heat treatment condition on the basis of the corrected mechanical characteristic value; and a heat treatment condition control step S8 of controlling a steel plate heating temperature in at least a rapid heating strip 9, in a continuous annealing facility, on the basis of the heat treatment condition determined by the heat treatment condition calculation step S4.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a control method for steel plate manufacturing equipment and steel plate manufacturing equipment. More specifically, it relates to a method for manufacturing steel plates used for structural materials for automobiles and the like. In particular, it enables reduction of the introduction cost of manufacturing equipment while suppressing the occurrence of material variations in the longitudinal direction of the steel strip.

Background Art

[0002] In the manufacture of thin steel plates for automobiles, continuously cast slabs are subjected to large amounts of processing by hot rolling and cold rolling until they reach the final plate thickness. Subsequently, in the annealing treatment that follows, recovery, recrystallization, and grain growth of the cold-worked structure are carried out, and furthermore, transformation structure control is performed to adjust the mechanical properties of the product in combination with the cooling treatment after annealing. In recent years, in order to achieve both weight reduction and collision safety of automobiles, higher-strength steel plates have been demanded. On the other hand, automobile body structural members are generally manufactured by press working, and products that achieve both high strength and high workability are demanded. In addition, as a material for automobile parts, zinc plating treatment is often carried out for the purpose of imparting rust prevention properties. In particular, from the viewpoint of press workability, alloyed hot-dip galvanized steel plates in which zinc and iron are alloyed by heating after plating treatment are widely adopted.

[0003] As a method for achieving high strength, utilization of a martensite structure obtained by rapidly cooling the austenite phase generated during annealing can be mentioned. However, since the martensite structure while cooled is brittle and difficult to handle, its toughness can be increased by a tempering treatment in which it is reheated. Therefore, a method is disclosed in Patent Document 1 in which rapid cooling is performed to below the martensite transformation temperature in the cooling zone on the outlet side of the annealing furnace and then reheating is carried out for tempering treatment.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

[0005] When performing a galvanizing process and an alloying process after tempering as in the method disclosed in Patent Document 1, generally, since the alloying temperature is higher than the tempering temperature, over-tempering occurs due to alloying heating, and the required strength cannot be obtained. Therefore, in the cooling zone in the annealing furnace, an alloying process is performed in a state where the austenite phase remains without transforming the steel strip into martensite, and after obtaining a martensite structure by rapid cooling, a tempering process is provided to perform tempering. A method for obtaining the characteristics of the target plating layer and the mechanical characteristics of the steel sheet is disclosed in Patent Document 2. However, in an actual continuous annealing process, since coils are joined together and heat treatment is performed continuously, it is necessary to change the operating conditions when the plate thickness, composition, or target mechanical characteristics change. If such a change in operating conditions is not reflected in the equipment, parts that cannot be manufactured under optimal conditions will occur before and after the joints of coils with different target mechanical characteristics, leading to a decrease in yield. In this regard, Patent Document 2 does not disclose a method for dealing with such problems.

[0006] Regarding the improvement of quality stability in the longitudinal direction, a technique for controlling the material of a steel sheet by a material prediction model using the operating parameters of a continuous annealing facility and the transformation rate information of the steel sheet obtained downstream of the continuous annealing facility as input data is disclosed in Patent Document 3. In this technique, it is said that material fluctuations can be suppressed by controlling the operating conditions of the manufacturing equipment on the downstream side of the position where the transformation rate information of the steel sheet called the material control zone is obtained. However, when a control delay occurs in the material control zone, the target mechanical characteristics cannot be obtained at the tip, and since no solution to this problem is presented, the effect is insufficient. Thus, the conventional technology has not been sufficient as a technology for enabling the steel sheet to be manufactured to have desired mechanical properties.

[0007] Therefore, the present invention has been made in view of the above problems, and an object thereof is to provide a control method for steel sheet manufacturing equipment and a steel sheet manufacturing equipment that can accurately obtain desired mechanical properties for the manufactured steel sheet.

Means for Solving the Problems

[0008] As a result of intensive studies to solve such problems, the present inventors have obtained the following findings. In the steel sheet manufacturing equipment as shown in FIG. 1 which is the object of the present invention, based on the heating temperature in the tempering zone of the preceding material, the mechanical properties, particularly the elongation value, when the succeeding material passes through the steel strip are predicted before and after the joint of the steel strip where the target mechanical properties change, and the heat treatment conditions of the continuous annealing equipment, particularly the steel sheet heating temperature in the rapid heating zone, are changed in real time before the succeeding material passes through so as to reduce the difference between the predicted value and the target elongation value. Even when the change in the heat treatment conditions in the tempering zone is delayed when the succeeding material passes through, by changing the heat treatment conditions in the rapid heating zone according to the heat treatment conditions, it was conceived that the target mechanical properties can be obtained 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 continuous annealing equipment for steel sheets having, in the steel sheet conveying direction, a heating zone, a soaking zone, a rapid heating zone, and a cooling zone in this order, An after-annealing heat treatment equipment having, at least in the downstream of the steel sheet conveying direction of the cooling zone, a rapid cooling zone and a tempering zone in this order, A temperature measuring device for the steel sheet installed in at least the rapid heating zone and the tempering zone, A method for controlling the steel sheet heating temperature in the continuous annealing equipment in the steel sheet manufacturing equipment including the above, A mechanical property prediction step of predicting mechanical property values including the elongation value and the hole expansion rate 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 value predicted in the mechanical property prediction step to the target mechanical property value of the steel sheet and determining heat treatment conditions based on the corrected mechanical property value; A heat treatment condition control step of controlling the heating temperature of the steel sheet at least in the rapid heating zone among the continuous annealing facilities based on the heat treatment conditions determined in the heat treatment condition calculation step; A control method for a steel sheet manufacturing facility, including the above. [2] In the heat treatment condition control step, based on the heat treatment conditions determined in the heat treatment condition calculation step, controlling the heating temperature of the steel sheet in the rapid heating zone and the steel sheet cooling conditions in the cooling zone, the control method for a steel sheet manufacturing facility according to [1] above. [3] In the steel sheet manufacturing facility, further comprising a transformation rate measuring device for measuring the austenite fraction of the steel sheet at at least one location from the outlet side of the soaking zone to the outlet side of the tempering zone in the steel sheet conveying direction. In the mechanical property prediction step, Based on the heating temperature of the steel sheet in the tempering zone measured by the temperature measuring device and the austenite fraction measured by the transformation rate measuring device, predicting the mechanical property value of the steel sheet, the control method for a steel sheet manufacturing facility according to [1] or [2] above. [4] A continuous annealing facility for steel sheets having a heating zone, a soaking zone, a rapid heating zone, and a cooling zone in this order in the steel sheet conveying direction, An after-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 conveying direction, A temperature measuring device for the steel sheet installed at least in the rapid heating zone and the tempering zone, A steel sheet manufacturing facility comprising: A mechanical property prediction unit that predicts mechanical property values including the elongation value and hole expansion property of the steel sheet based on the heating temperature of the steel sheet in the tempering zone measured by the temperature measuring device; A heat treatment condition calculation unit that corrects the mechanical property value predicted by the mechanical property prediction unit to the target mechanical property value of the steel sheet and determines heat treatment conditions based on the corrected mechanical property value; Based on the heat treatment conditions determined by the heat treatment condition calculation unit, among the continuous annealing equipment, a heat treatment condition control unit that controls the heating temperature of the steel sheet at least in the rapid heating zone, A steel sheet manufacturing facility further comprising [5] In the heat treatment condition control unit, based on the heat treatment conditions determined by the heat treatment condition calculation unit, the steel sheet manufacturing facility according to [4] above, which controls the heating temperature of the steel sheet in the rapid heating zone and the steel sheet cooling conditions in the cooling zone. [6] Further comprising a transformation rate measuring device that measures the austenite fraction of the steel sheet at at least one location from the soaking zone outlet side to the tempering zone outlet side in the steel sheet conveyance direction, In the mechanical property prediction unit, Based on the heating temperature of the steel sheet in the tempering zone measured by the temperature measuring device and the austenite fraction measured by the transformation rate measuring device, the steel sheet manufacturing facility according to [4] or [5] above, which predicts the mechanical property values of the steel sheet.

Effect of the Invention

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

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0012] <Equipment Configuration> Figure 1 is a schematic diagram of a steel sheet manufacturing facility 1 according to an embodiment of the present invention. As shown in Figure 1, a steel sheet S wound in a coil shape in a previous process is unwound by a payoff reel 2 and enters a looper 4 along the steel sheet conveyance direction (see reference sign X in Figure 1). The looper 4 is a facility for securing an extra length of the steel sheet in order to continuously pass the steel sheet S when joining the steel sheets S (coils) 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 on the inlet side of the continuous annealing facility 5. Alternatively, further, a preheating zone 6 using combustion exhaust gas generated in the heating zone 7 may be installed in front of the heating zone 7. The continuous annealing facility 5 is configured, in this order following the heating zone 7, with 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 required cooling amount and the sheet passing speed according to the required mechanical properties, or may have a two-stage configuration including the first cooling zone 10A and the second cooling zone 10B as shown in Figure 1.

[0013] Downstream in the steel sheet conveyance direction X of the cooling zone 10, an annealing post-heat treatment facility 11 configured, in this order, with a rapid cooling zone 15 and a tempering zone 16 is installed. When the manufactured steel sheet S is a plated steel sheet (alloyed hot-dip galvanized steel sheet), the annealing post-heat treatment facility 11 may be a steel sheet plating facility (alloyed hot-dip galvanizing facility), and may have, in the steel sheet conveyance direction X, a hot-dip galvanizing immersion zone 12 and a plating alloying zone 13 in this order in front of the rapid cooling zone 15. When the annealing post-heat treatment facility 11 is a steel sheet plating facility (alloyed hot-dip galvanizing facility), the steel sheet manufacturing facility 1 of the present invention may be a hot-dip galvanized steel sheet manufacturing facility or an alloyed hot-dip galvanized steel sheet manufacturing facility. At this time, for the purpose of ensuring the time required for the progress of the alloying reaction, the annealing post-heat treatment facility 11 may include a heat preservation zone 14. The tempering zone 16 is configured, in this order, with a heating device (tempering zone heating device) 17, a heat preservation device (tempering zone heat preservation device) 18, and a cooling device (tempering zone cooling device) 19.

[0014] In the steel sheet manufacturing facility 1 of the present invention, a temperature measuring device 20 for measuring the temperature of the steel sheet surface is installed at least in the rapid heating zone 9 and the tempering zone 16, so that the temperature change of the steel sheet can be monitored. Further, a transformation rate measuring device 21 for measuring the austenite fraction of the steel sheet 1 may be provided at at least one location from the outlet side of the soaking zone 8 to the outlet side of the tempering zone 16.

[0015] Further, the steel sheet manufacturing facility 1 of the present invention may have a management device 30. The management device 30 includes a mechanical property prediction unit 31 that predicts mechanical property values including the elongation value and hole expansion rate of the steel sheet S based on the steel sheet heating temperature in the tempering zone 16 measured by the temperature measuring device 20, and corrects the mechanical property values predicted by the mechanical property prediction unit 31 to the target mechanical property values (target elongation value, target hole expansion rate, etc.) of the steel sheet S, and a heat treatment condition calculation unit 32 that 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 among the facilities constituting the continuous annealing facility 5 based on the heat treatment conditions determined by the heat treatment condition calculation unit 32. By having the mechanical property prediction unit 31, the heat treatment condition calculation unit 32, and the heat treatment condition control unit 33, the steel sheet manufacturing facility 1 of the present invention can suppress fluctuations in the mechanical properties of the obtained 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 room temperature to about 100°C passes through the looper 4 and enters the preheating zone 6. The preheating zone 6 is used to improve the energy efficiency in the heating zone 7 and soaking zone 8 described later. Even if the steel plate manufacturing facility 1 has the preheating zone 6, the steel plate S such as thin steel plate may start to be heated in the next heating zone 7 in the steel plate conveying direction X without being heated in the preheating zone 6. When the steel plate S is heated in the preheating zone 6, the steel plate S such as thin steel plate is heated to about 200°C. As for the heating method of the preheating zone 6, a method using the high-temperature exhaust gas generated in the next heating zone 7 is conceivable, but it is not particularly limited as long as the target temperature is reached.

[0018] (2) Heating zone 7 Subsequently, the steel plate S enters the heating zone 7, and the steel plate temperature is heated to 600 - 700°C, which is about the same as the annealing temperature (soaking temperature) to ensure the maximum holding time in the soaking zone. In the heating zone 7, in addition to having high heating capacity and being able to reduce the furnace volume, when plating is applied to the steel plate S, the oxidation-reduction reaction on the steel plate surface can be flexibly controlled to ensure plating properties in subsequent processes. Therefore, it is preferable to adopt the direct-fired heating furnace method. By this method, it is possible to heat to the target temperature in a short time, and it is easy to control the state of the steel plate surface.

[0019] (3) Soaking zone 8 In the subsequent soaking zone 8, the steel plate heated in the heating zone 7 is held or gently heated. As the heating method of the soaking zone 8, it is preferable to adopt the radiant heating (radiant tube heating) method by gas combustion from the viewpoints of efficiency and high heating uniformity, but it is not particularly limited. In the present invention, the final annealing temperature is determined in the subsequent rapid heating zone 9. When the fine 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 the α-phase to the γ-phase proceeds rapidly, making it difficult to obtain the target fraction of the structure. Therefore, in the soaking zone 8, recrystallization progress of the α-phase and slow heating or holding at a temperature above the A1 transformation point are carried out. By holding or slowly heating in the temperature range below the A1 transformation point, the residence time in the recrystallization temperature range can be ensured, and the remaining unrecrystallized α-phase can be suppressed. At this time, if the residence time in the recrystallization temperature range is too short, the progress of the recrystallization of the α-phase becomes insufficient. On the contrary, if it is too long, the crystal grains become coarse and the mechanical properties deteriorate. Therefore, the residence time in this temperature range is preferably 20 to 60 seconds. Even when slowly heating within the soaking zone 8, it is preferable that the residence time in the recrystallization temperature range is ensured for the above time (20 to 60 seconds). Further, after the recrystallization time is ensured, if it is below the target annealing temperature described later, it may be slowly heated to a temperature above the A1 transformation point. However, the heating rate during slow heating within the recrystallization temperature range is preferably 5 °C / s or less. This is because if heating is performed at more than 5 °C / s, there is a concern that the remaining unrecrystallized α-phase cannot be suppressed.

[0020] (4) Rapid heating zone 9 In the above-mentioned soaking zone 8, after the recrystallization of the α-phase has proceeded, in the rapid heating zone 9, it is necessary to heat the steel sheet S to the target annealing temperature set in the temperature range higher than the A1 transformation point of the steel sheet S and lower than the A3 transformation point. Since the reaching temperature at this time has a great influence on the final material, as the heating method, it is preferable to select an induction heating method with a fast response to temperature control. Further, since it is heated to a high temperature exceeding the A1 transformation point, it exceeds the Curie point at which the magnetic properties of the steel sheet S change, so it is preferable to use a transverse induction heating device. Since it is held in the recrystallization range of the α-phase in the above-mentioned soaking zone 8, if the heating takes more time than necessary, coarsening of the α grains may occur. Therefore, the heating rate is preferably 10 °C / s or more. Also, it is desirable to make the equipment length shorter and suppress the construction cost. Considering the equipment cost in this way, it is more preferable to set it at 20 °C / s or more. On the one hand, when the heating rate is increased to increase the temperature rise, local high-temperature parts may occur in the width direction, impairing the material uniformity of the entire steel plate. Therefore, the heating rate is preferably 200 °C / s or less. However, when a thin steel plate is rapidly heated, the steel plate may undergo buckling deformation due to thermal stress, which may cause conveyance troubles. Considering the operational stability, the heating rate is more preferably 100 °C / s or less. When rapidly heating to the target annealing temperature by an induction heating device or the like, it may happen that the transformation from the α-phase to the γ-phase has not reached the equilibrium state immediately after heating. At this time, when holding near the target annealing temperature, even if it is isothermal holding, the transformation from the α-phase to the γ-phase may further proceed in some cases. Since the holding time varies depending on the line speed, there is a concern that material control may become complicated due to holding near the target annealing temperature. Therefore, after reaching the target annealing temperature, it is desirable to enter the cooling zone 10 as soon as possible, preferably start cooling within 10 seconds, and more preferably start cooling 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. In the case of adding to an existing furnace, it may be installed at the connection part between the soaking zone 8 and the cooling zone 10. Also, when the soaking zone 8 is divided into a plurality of compartments, a plurality of rapid heating devices may be installed other than immediately before the cooling zone 10. For example, when the soaking zone 8 is divided into two compartments, holding is performed in the upstream compartment in the recrystallization temperature range, and holding is performed in the downstream compartment in a high-temperature range to promote the reduction reaction on the steel plate surface. In such a case, since there is a difference in the optimum temperature range for each compartment, the first rapid heating device installed after the upstream soaking zone 8 heats from the low-temperature compartment to the temperature of the high-temperature compartment, and after the downstream soaking zone 8 and immediately before entering the cooling zone 10, a second rapid heating device for the purpose of temperature adjustment for material control may be installed.

[0021] (5) Cooling zone 10 (First cooling zone 10A, Second cooling zone 10B) After being heated to the target annealing temperature by the rapid heating zone 9, the steel sheet S is conveyed to the cooling zone 10 for cooling. For the reason of preventing ferrite transformation during cooling due to insufficient cooling rate and preventing rapid bainite transformation after cooling stop due to excessive cooling rate, the range of the cooling rate is preferably 5 °C / s or more and 30 °C / s or less. In order not to cause martensite transformation upstream of the plating process and to transform a part of the austenite phase into the bainite phase, the cooling stop temperature is preferably not less than the martensite transformation start temperature and not more than 550 °C, and more preferably within the range of 450 °C or more and 550 °C or less. Also, in order to allow sufficient progress of bainite transformation, it is preferable to hold for 1 second or more and 100 seconds or less after reaching the cooling stop temperature, and more preferably to hold for 20 seconds or more and 50 seconds or less. As the cooling method, gas jet cooling in which a compressed gas jet collides, roll cooling in which cooling is performed by contact with a roll through which a refrigerant is passed, water cooling by a water jet, mist cooling in which compressed gas and fine water droplets are mixed, etc. can be considered, but it is not particularly limited. In the present embodiment described later, gas jet cooling is adopted which can ensure the target cooling rate, does not cause unstable temperature changes due to boiling, and can control the cooling stop temperature with high precision. Also, as shown in FIG. 1, when slow cooling or heat retention is performed to further promote bainite transformation after cooling, the cooling zone 10 may be divided into two parts: a first cooling zone (rapid cooling zone) 10A and a second cooling zone (slow cooling / heat retention zone) 10B.

[0022] (6) Plating immersion zone 12 (molten zinc plating immersion zone), plating alloying zone 13 and heat retention zone 14 After the structure is controlled in the cooling zone 10, the steel sheet S is conveyed to the post-annealing heat treatment facility 11. When the steel sheet S is subjected to a plating treatment, the post-annealing heat treatment facility 11 may be a steel sheet plating facility, and the steel sheet S is conveyed to this steel sheet plating facility, and a plating treatment (for example, zinc plating treatment) is performed in the plating immersion zone 12. In the present invention, as the plating method, a hot-dip galvanizing method may be adopted in which a steel sheet is immersed in a galvanizing bath storing molten zinc. In addition, in order to control the amount of zinc adhered after passing through the plating bath, the post-annealing heat treatment facility (steel sheet plating facility) 11 may be provided with a device for scraping off excess molten zinc. Downstream of the process of the plating immersion zone 12, a plating alloying zone 13 for promoting the Zn-Fe alloying reaction may be provided. Since the temperature of the steel sheet at this time has dropped to about 430°C, the temperature is raised to the temperature required for the alloying reaction (about 500°C) in the plating alloying zone 13. As the heating method in the plating alloying zone 13, a gas heating method using combustion exhaust gas, induction heating, an electric heating method, etc. can be considered, but it is not particularly limited. In the present invention, an induction heating method that has little influence on the plating surface and enables fine output control may be adopted. For the purpose of ensuring the time required for the progress of the alloying reaction, a heat preservation zone 14 may be installed on the downstream side of the plating alloying zone 13.

[0023] (7) Rapid cooling zone 15 When the steel sheet S is plated (when the post-annealing heat treatment facility 11 is used as a steel sheet plating facility), following the plating alloying zone 13 or the heat preservation zone 14, after the plating alloying reaction is completed, a rapid cooling zone 15 for transforming the untransformed austenite phase into a martensite structure is installed. When the steel sheet S is not plated, the rapid cooling zone 15 may be installed downstream of the cooling zone 10 in the steel sheet conveying direction X. As the cooling method in the rapid cooling zone 15, gas jet cooling in which a compressed gas jet is collided, water cooling by a water jet, mist cooling in which compressed gas and fine water droplets are mixed, etc. can be considered, but it is not particularly limited. In the present invention, in order to realize rapid cooling and control the cooling stop temperature with high precision, a combination of mist cooling and gas jet cooling may be used. When the steel sheet S is plated, the steel sheet S that has passed through the plating alloying zone 13 or the heat preservation zone 14 and has reached about 350 to 450°C is cooled in the rapid cooling zone 15 to below the martensite transformation start temperature. In order to obtain a desired fraction of martensite structure while preventing excessive bainite transformation, the cooling rate at this time is preferably 50°C / s or more. On one hand, there is no specific upper limit set for the cooling rate. However, if excessive rapid cooling is carried out, the flatness of the steel plate may be disrupted due to thermal deformation during cooling. Therefore, the cooling rate is preferably 1000 °C / s or less. Also, the cooling stop temperature is preferably 200 °C or less in order to prevent self-tempering after cooling.

[0024] (8) Tempering zone 16 After forming a martensite structure in the rapid cooling zone 15, tempering is carried out in the tempering zone 16. The tempering zone 16 is configured in the order of a heating device (tempering zone heating device) 17, a heat preservation device (tempering zone heat preservation device) 18, and a cooling device (tempering zone cooling device) 19 in the steel plate conveyance direction X from the inlet. The heating temperature in the heating device 17 is preferably 250 °C or more and 500 °C or less, more preferably 300 °C or more and 400 °C or less, so as to obtain the effect of improving toughness by tempering while maintaining the martensite structure. In the heat preservation device 18, the temperature of the steel plate heated in the heating device 17 is maintained, and the holding time in the heat preservation device 18 is preferably 20 seconds or more and 100 seconds or less, more preferably 30 seconds or more and 60 seconds or less, in order to sufficiently obtain the effect of tempering while preventing the expansion of an excessive line length. Thereafter, in the cooling device 19, the steel plate S is cooled to room temperature. With this configuration, an appropriate tempering treatment can be performed, and the target mechanical properties can be obtained. The heating in the heating device 17 can be selected from methods such as gas burners, electric heating, and induction heating. In the present invention, an induction heating method may be adopted from the perspective of controllability. The heat preservation device 18 preferably adopts a radiation heating method from the heater body by an electric heater and the furnace wall from the perspective of temperature uniformity inside the device, but is not particularly limited. In the steady state, control may be performed so that the furnace temperature is approximately the same as the target tempering temperature. The cooling device 19 is preferably a mist cooling method from the perspective of cooling efficiency, and it is also preferable to install a gas jet device to remove the moisture adhering to the surface during cooling, but it is not particularly limited as long as it can be adjusted to the target cooling rate.

[0025] (9) Temperature measuring device 20 Among the above-mentioned respective zones, at least in the rapid heating zone 9 and the tempering zone 16, a temperature measuring device (thermometer) 20 for measuring the surface temperature of the steel plate is installed. By means of these temperature measuring devices 20, it becomes possible to know the temperature history of the steel plate S during heat treatment. When installing in a facility with a long facility length such as the soaking zone 8, a thermometer may also be installed in the zone to confirm the temperature history in the middle. Specifically, the installation positions of the above-mentioned thermometer in each zone are, for example, in the soaking zone 8, the soaking zone inlet (= heating zone outlet), the middle of the soaking zone, and the soaking zone outlet (= rapid heating zone inlet). Further, the installation positions of the above-mentioned thermometer in each zone are, in the rapid heating zone 9, the rapid heating zone inlet (= soaking zone outlet) and the rapid heating zone outlet (= cooling zone inlet), and in the tempering zone 16, the tempering zone inlet, the middle of the tempering zone, and the tempering zone outlet. The temperature measurement method by the temperature measuring device 20 is not particularly limited, but it is preferably a radiation thermometer that measures the temperature by sensing the infrared rays emitted by the steel plate. Since the radiation thermometer is affected by the reflected light of the infrared rays emitted by the surrounding furnace body, a cover may be provided between the measurement part and the detection part provided in the radiation thermometer. Further, since the radiation thermometer is also affected by the emissivity of the steel plate surface, as a temperature measurement method, a multiple reflection type measurement method using the wedge-shaped space between the in-furnace conveying roll and the steel plate S may be adopted.

[0026] (10) Transformation rate measuring device 21 Preferably, at least one transformation rate measuring device 21 for measuring the austenite fraction of the steel plate S is provided from the outlet side of the soaking zone 8 to the outlet side of the tempering zone 16. This is because, based on the measured austenite fraction, by predicting the mechanical property values (for example, elongation value, hole expansion rate) of the steel plate S, the prediction accuracy of the mechanical property values of the steel plate S is improved compared to predicting the mechanical property values such as the elongation value and hole expansion rate of the steel plate S only from the heating temperature of the steel plate in the tempering zone 16. By means of this transformation rate measuring device 21, information on the fractions (transformation rates) of the α-phase and γ-phase of the steel plate after reaching the target annealing temperature can be obtained. This transformation rate measuring device 21 is provided for the purpose of adjusting the operating conditions of the continuous annealing facility 5 based on the measured austenite fraction, enabling the stable acquisition of desired mechanical properties. The method for measuring the transformation rate is not particularly limited. However, the transformation rate measuring device 21 is preferably a magnetic detector, that is, a device for measuring the magnetic transformation rate of the steel sheet S (steel strip), and can measure the austenite fraction 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 passing through the steel sheet S. As another method, for example, a method applying the X-ray diffraction method may be adopted. In the method applying the X-ray diffraction method, since the γ-phase and the α-phase generate diffraction peaks at specific angles when the steel sheet is irradiated with X-rays due to the difference in crystal structure, the austenite fraction can be quantified based on the diffraction peak intensity.

[0027] (Mechanical property values (elongation value, hole expansion rate)) As the mechanical property value of the steel sheet S of the present invention, the elongation value is determined based on, for example, the test method of JIS Z2241 (2011). The sample for measuring the elongation value is preferably collected from the steel sheet that has been cut by the tempering zone cooling device 19 in the present invention and immediately before coiler winding. Also, for each coil, the sample is collected from the front end and the tail end of the coil during operation. As the mechanical property value of the steel sheet S of the present invention, the hole expansion rate is determined based on, for example, the test method of JIS Z2256 (2010). The sample for measuring the hole expansion rate is preferably collected from the steel sheet that has been cut immediately before coiler winding. Also, for each coil, the sample is collected from the front end and the tail end of the coil during operation.

[0028] <Control method> FIG. 2 is a flowchart for explaining the control method of the steel sheet manufacturing facility in the present invention. The configurations of the mechanical property prediction unit 31, the heat treatment condition calculation unit 32, and the 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 of FIG. 2.

[0029] (1) Mechanical property prediction unit 31, mechanical property prediction step In the steel sheet manufacturing facility 1, a new steel sheet S is manufactured (step S1), and at the joint portion of the steel sheet S (coil), that is, at the connection position between the preceding material and the succeeding material, it is determined whether the coil requires a change in manufacturing conditions (step S2). If no change in manufacturing conditions is required, the manufacturing conditions are not changed, and the steel sheet manufacturing facility 1 continues to manufacture the steel sheet S. The determination in this step S2 may be performed by a control unit (not shown) in the management device 30 of the steel sheet manufacturing facility 1, or may be performed by another control unit (not shown) that the steel sheet manufacturing facility 1 has. For example, when the plate thickness, steel component composition, and target mechanical properties are different between the preceding material and the succeeding material, the management device 30 determines that a change in manufacturing conditions is required. When a change in manufacturing conditions is required, in the mechanical property prediction step S3, the mechanical property prediction unit 31 predicts the mechanical property values of the steel sheet S (succeeding material) based on the operating conditions (steel sheet heating temperature) in the tempering zone (reheating zone) 16. Examples of the mechanical property values include elongation values and hole expansion rates. Examples of the operating conditions input to the mechanical property prediction unit 31 include the operating conditions of equipment downstream of the rapid heating zone 9. Based on these operating conditions, the mechanical properties when the succeeding material has completed the process up to the tempering zone (reheating zone) 16 are predicted. At this time, by further adding the austenite fraction result measured by the transformation rate measuring device 21 to the operating conditions and performing the prediction of the mechanical property values based on this austenite fraction and the operating conditions, the prediction accuracy of the mechanical property values is improved. Since only temperature measurement indirectly predicts the structure, by further directly measuring the austenite fraction, the prediction accuracy of the mechanical properties after passing through the heat treatment process is improved. As a method for predicting mechanical properties performed by the mechanical property prediction unit 31, for elongation values and hole expansion rates, for example, an empirical formula calculated by performing regression analysis based on the results of heat treatment experiments, or a method using an elongation value prediction database constructed by a data science method based on operating conditions and material measurement results can be considered, but it is not particularly limited. In the present invention, from the viewpoint of improving the prediction accuracy for new varieties, a regression analysis method based on the experimental results of the heat treatment of each steel plate can be used. As this procedure, heat treatment simulating an actual machine is carried out offline or in an experiment. At this time, for the elongation value, regression analysis is performed based on the results of the test method of JIS Z2241 (2011), and for the hole expansion rate, regression analysis is performed based on the results of the test method of JIS Z2256 (2010) to predict the elongation value and the hole expansion rate. The mechanical property prediction unit 31 can record data in the storage unit 34 having the above-mentioned elongation value prediction database, or predict the elongation value and the like of the steel plate 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 value of the steel plate S (subsequent material) predicted by the mechanical property prediction unit 31 in the mechanical property prediction step S3 is corrected to the target mechanical property value, and the heat treatment conditions in the continuous annealing facility 5 are calculated based on the corrected mechanical property value. The correction here refers to changing the mechanical property value for calculating the heat treatment conditions from the predicted mechanical property value to a preset target mechanical property value. At this time, when the heat treatment condition calculation unit 32 determines that it is possible to correct to the target mechanical property value only by changing the steel plate heating condition in the rapid heating zone 9 (step S5), the heat treatment condition calculation unit 32 calculates and determines the condition of the steel plate heating temperature in the rapid heating zone 9 (step S6a). On the other hand, when it is determined that the target mechanical property value cannot be corrected only by changing the steel plate heating temperature in the rapid heating zone 9 (step S5), the heat treatment condition calculation unit 32 calculates the steel plate cooling condition in the cooling zone 10 in addition to the heating temperature in the rapid heating zone 9 (step S6b). As a method for determining the heat treatment conditions (the steel plate heating temperature in the rapid heating zone 9 and the steel plate cooling conditions in the cooling zone 10), for example, a method using an empirical formula calculated by performing regression analysis based on the results of experiments, or a heat treatment condition database constructed by a method based on data science using operating conditions and material measurement results can be considered, but it is not particularly limited. In the present invention, from the viewpoint of prediction accuracy for new varieties, a regression analysis method based on the experimental results of heat treatment of each steel plate can be used. As this procedure, heat treatment simulating an actual machine can be performed offline or in an experiment, and based on the results, regression analysis can be performed to predict elongation values, hole expansion rates, etc. The steel plate cooling conditions in the cooling zone 10 are not particularly limited, and examples include the cooling stop temperature and the cooling rate in the cooling zone 10. These cooling stop temperatures and cooling rates can be obtained by measuring with a thermometer installed in the cooling zone 10. The heat treatment condition calculation unit 32 can record data in the storage unit 34 having the above heat treatment condition database, and can calculate and determine the heat treatment conditions of the steel plate 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 plate heating temperature in the rapid heating zone 9 or further the steel plate cooling conditions in the cooling zone 10 based on the heat treatment conditions determined by the heat treatment condition calculation unit 32. Based on the above heat treatment conditions, not only the steel plate heating temperature in the rapid heating zone 9 but also the steel plate cooling conditions in the cooling zone 10 are controlled, so that the mechanical properties (elongation value, hole expansion rate) of the steel plate S (rearward zone) can be controlled more accurately.

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

[0033] By performing the control described with reference to FIG. 2, for example, even when there is a control delay and there is a concern that the operating conditions in the tempering zone 16 are different from the target tempering conditions of the subsequent material and the desired mechanical property values cannot be obtained, the information on the steel plate heating temperature is input to the mechanical property prediction unit 31, and the heat treatment conditions in the continuous annealing facility 5 necessary for correcting to the target mechanical property values are calculated by the heat treatment condition calculation unit 32, and the heat treatment conditions in the rapid cooling zone 9 and / or the cooling zone 10 are controlled by the heat treatment condition control unit 33 to cancel the control delay in the tempering zone 16. In this way, for the obtained steel plate, the variation of the mechanical properties from the desired mechanical properties can be reduced, and compared with the conventional steel plate manufacturing facility, it becomes possible to stably obtain the target mechanical properties over the longitudinal direction.

Example

[0034] Using the steel sheet manufacturing equipment according to the above-described embodiment of the present invention, thin steel sheets (coils, steel strips) were manufactured. To examine the variation in mechanical properties and the yield of the products, three types of products with strength levels of 780 MPa grade, 980 MPa grade, and 1180 MPa grade were continuously manufactured. Five pieces of each strength were continuously passed through the plate, and a total of 150 pieces were manufactured in 10 sets, with 15 pieces (3 types) in each set. The plate thickness was in the range of 1.0 to 2.0 mm, and the manufacturing order was determined so that the plate thickness was almost constant within the set, and sets with gradually changing plate thickness were connected and manufactured. In addition, products of the same strength included in the same set were manufactured using slabs cast in different lots in the continuous casting machine as materials. More specifically, although within the manufacturing management range, the chemical components of each slab varied and the transformation behavior was not uniform.

[0035] Each slab was hot-rolled, pickled, annealed if necessary, cold-rolled by a conventional method, then heat-treated using a conventional annealing facility or the annealing facility of the present invention, and thereafter post-treatments such as cooling and plating were carried out. The variation in characteristics between products of the same strength was examined using the measurement results of samples taken at the 10 m position from the coil tip of the final product as representative values. Also, at the connection position between products of different strengths, analysis samples were taken every 10 m in the range of 100 m before and after the connection position, the region where the mechanical properties did not meet the shipping standards was examined, and the ratio of the length that could be shipped to the original coil length was calculated as the yield of the connection part. The yield of the connection part was considered qualified if it was 90% or more. Note that the tensile test pieces were JIS No. 5, and the tensile test was carried out in accordance with JIS Z2241 (2011). At each strength of 780 MPa grade, 980 MPa grade, and 1180 MPa grade, the required strength ranges were 780 MPa or more, 980 MPa or more, and 1180 MPa or more, respectively, and the ductility was 17% or more, 15% or more, and 12% or more. Also, the hole expansion test was carried out in accordance with JIS Z2256 (2010). The required hole expansion ratios were 65% or more, 50% or more, and 40% or more at each strength of 780 MPa grade, 980 MPa grade, and 1180 MPa grade.

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

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

[0038]

Table 1

[0039] Comparative Example 1 is an example in which products were manufactured from the above materials using a conventional CGL composed of a preheating zone, a heating zone, a soaking zone, a cooling zone, a plating zone, a plating alloying zone, and a final cooling zone described in Citation 1. Comparative Example 2 is an example in which a tempering zone was provided after the plating alloying zone 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 of Comparative Example 2. Comparative Example 4 is an example in which a transformation rate measuring device was installed downstream of the induction heating device of Comparative Example 3. In these comparative examples, even when a tempering zone (reheating zone) was used, the control of the continuous annealing facility based on the steel plate heating temperature in the tempering zone was set as "not implemented".

[0040] As shown in Table 1, in Comparative Example 1, the TS within products of the same strength varied greatly, and there were also those that fell below the lower limit value for each grade. Also, since tempering in the tempering zone was not carried out, it was not possible to manufacture products that met the ductility acceptance criteria, and the yield at the connection part could not be calculated. In Table 1, it is indicated as "-".

[0041] In Comparative Example 2, by performing tempering in the tempering zone, the ductility was improved compared to Comparative Example 1, and it became possible to manufacture products within the acceptance criteria. However, due to the occurrence of heating temperature in the annealing furnace and control delay in the tempering zone, there were many parts where the control of the steel plate temperature deviated from the target. Furthermore, there was a large variation in mechanical properties even within the coil, and a wide area that did not meet the material test criteria occurred at the connection part, resulting in a low yield at the connection part.

[0042] In Comparative Example 3, the control range of the annealing temperature was widened by the induction heating device, and no out-of-TS occurred. However, since the control of the continuous annealing facility based on the heating temperature of the steel plate in the tempering zone was not performed, the variation in ductility was large, and those that did not meet the acceptance criteria occurred. Also, the yield at the connection part was not sufficiently improved.

[0043] In Comparative Example 4, by using a transformation ratio measuring device, the controllability of the annealing temperature by the induction heating device was improved, and the variation in TS was further improved. However, since the control of the continuous annealing facility based on the heating temperature of the steel plate in the tempering zone was not performed, the variation in ductility was not improved. Also, the yield at the connection part was not improved.

[0044] For these comparative examples, Example 1 is the result of manufacturing products from the above materials using the control method of the steel plate manufacturing facility in the present invention. Example 3 is an example in which a transformation ratio measuring device is installed downstream of the induction heating device of Example 1. In Examples 1, 2, and 3, the control of the continuous annealing facility based on the heating temperature of the steel plate in the tempering zone was "performed". As a result, at the connection part of the coil, by controlling the continuous annealing facility based on the heating temperature of the steel plate in the tempering zone, almost all of the products at the connection part were determined to be shippable except in the vicinity of the welded part. Furthermore, the variation in TS was further improved by controlling the annealing temperature based on the austenite fraction measured by the transformation ratio measuring device.

Explanation of symbols

[0045] S Steel plate X Steel plate conveying direction 1 Steel plate manufacturing equipment 2 Payoff reel 3 Welder 4 Looper 5 Continuous annealing equipment 6 Preheating 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 plate plating equipment) 12 Plating immersion zone 13 Plating alloying zone 14 Heat preservation zone 15 Rapid cooling zone 16 Tempering zone 17 Tempering zone heating device 18 Tempering zone heat preservation device 19 Tempering zone cooling device 20 Temperature measurement device 21 Transformation rate measurement 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 continuous annealing facility for steel sheets having, in the steel sheet conveyance direction, a heating zone, a soaking zone, a rapid heating zone, and a cooling zone in this order, and an after-annealing heat treatment facility having, downstream in the steel sheet conveyance direction of the cooling zone, at least a rapid cooling zone and a tempering zone in this order, and a temperature measuring device for the steel sheet installed in at least the rapid heating zone and the tempering zone, A method for controlling the heating temperature of a steel sheet in the continuous annealing facility in a steel sheet manufacturing facility comprising: a mechanical property prediction step of predicting mechanical property values including the elongation value and hole expansion rate of the steel sheet based on the heating temperature of the steel sheet 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 the 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 the heating temperature of the steel sheet in at least the rapid heating zone among the continuous annealing facilities based on the heat treatment conditions determined in the heat treatment condition calculation step; A control method for a steel sheet manufacturing facility, including the above steps.

2. In the heat treatment condition control step, based on the heat treatment conditions determined in the heat treatment condition calculation step, controlling the heating temperature of the steel sheet in the rapid heating zone and the steel sheet cooling conditions in the cooling zone. The control method for a steel sheet manufacturing facility according to Claim 1.

3. In the steel sheet manufacturing facility, further comprising a transformation rate measuring device for measuring the austenite fraction of the steel sheet at at least one location from the soaking zone outlet side to the tempering zone outlet side in the steel sheet conveyance direction, In the mechanical property prediction step, Predicting the mechanical property values of the steel sheet based on the heating temperature of the steel sheet in the tempering zone measured by the temperature measuring device and the austenite fraction measured by the transformation rate measuring device. The control method for a steel sheet manufacturing facility according to Claim 1 or 2.

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

5. The steel sheet manufacturing facility according to claim 4, wherein in the heat treatment condition control unit, the heating temperature of the steel sheet in the rapid heating zone and the steel sheet cooling condition in the cooling zone are controlled based on the heat treatment conditions determined by the heat treatment condition calculation unit.

6. Further comprising a transformation rate measuring device that measures the austenite fraction of the steel sheet at at least one location from the out-of soaking zone side to the out-of tempering zone side in the steel sheet conveying direction; In the mechanical property prediction unit, The steel sheet manufacturing facility according to claim 4 or 5, wherein the mechanical property value of the steel sheet is predicted based on the heating temperature of the steel sheet 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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