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 plate 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 uniform heating strip 8, 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 method for manufacturing high-strength steel sheets used for automotive structural materials and the like, and in particular, it enables reduction of the introduction cost of manufacturing equipment while suppressing the occurrence of material variations.

Background Art

[0002] In the manufacture of thin steel sheets for automobiles, continuously cast slabs are subjected to large deformations by hot rolling and cold rolling until they reach the final plate thickness. Subsequently, in the annealing process, the 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 process after annealing. In recent years, in order to achieve both weight reduction and collision safety of automobiles, higher-strength steel sheets have been demanded. On the other hand, automotive body structural members are generally manufactured by press working, and products with both high strength and high workability are required. In addition, as a material for automotive parts, zinc plating treatment is often performed for the purpose of providing rust prevention properties. In particular, from the viewpoint of press workability, alloyed hot-dip galvanized steel sheets 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 in the cooled state is brittle and difficult to handle, its toughness can be increased by a tempering process of reheating. Therefore, a method is disclosed in Patent Document 1 in which rapid cooling is performed to a temperature below the martensite transformation temperature in the cooling zone on the outlet side of the annealing furnace and then reheating is performed for tempering.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] When performing a zinc plating treatment and an alloying treatment 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 treatment 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 step of performing tempering is provided, whereby 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, and 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 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 apparatus 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 making the manufactured steel sheet have desired mechanical characteristics.

[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 steel sheet manufacturing equipment that enable desired mechanical properties to be accurately obtained 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, before and after the joint of the steel strip where the target mechanical properties change, based on the heating temperature in the tempering zone of the preceding material, the mechanical properties when the succeeding material passes through the plate, particularly the elongation value, are predicted, and the heat treatment conditions of the continuous annealing equipment, particularly the steel sheet heating temperature in the soaking zone, are changed before the succeeding material passes through so as to reduce the difference between the predicted value and the target elongation value. Thus, even when the change in the heat treatment conditions in the tempering zone is delayed, by changing the heat treatment conditions in the soaking zone in accordance with 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 concepts, and has the following features. [1] A continuous annealing facility for steel sheets having, in the steel sheet conveyance direction, a heating zone, a soaking zone, and a cooling zone in this order, An after-annealing heat treatment facility having, downstream of the cooling zone in the steel sheet conveyance direction, at least a rapid cooling zone and a tempering zone in this order, A temperature measuring device for the steel sheet installed in at least the soaking zone and the tempering zone, A method for controlling the steel sheet heating temperature in the continuous annealing facility in a steel sheet manufacturing facility including: 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 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 the target mechanical property values of the steel sheet and determining heat treatment conditions based on the corrected mechanical property values, Based on the heat treatment conditions determined in the heat treatment condition calculation step, a heat treatment condition control step for controlling the steel plate heating temperature at least in the soaking zone among the continuous annealing facilities, A control method for a steel plate manufacturing facility including [2] In the heat treatment condition control step, based on the heat treatment conditions determined in the heat treatment condition calculation step, the control method for a steel plate manufacturing facility according to [1], wherein the steel plate heating temperature in the soaking zone and the steel plate cooling conditions in the cooling zone are controlled. [3] In the steel plate manufacturing facility, further comprising a transformation rate measuring device for measuring the austenite fraction of the steel plate at at least one location from the outlet side of the soaking zone to the outlet side of the tempering zone in the steel plate conveyance direction, In the mechanical property prediction step, 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, the control method for a steel plate manufacturing facility according to [1] or [2], wherein the mechanical property value of the steel plate is predicted. [4] A continuous annealing facility for a steel plate having a heating zone, a soaking zone, and a cooling zone in this order in the steel plate conveyance direction, An after-annealing heat treatment facility having at least a rapid cooling zone and a tempering zone in this order downstream of the steel plate conveyance direction in the cooling zone, A temperature measuring device for the steel plate installed at least in the soaking zone and the tempering zone, A steel plate manufacturing facility comprising: A mechanical property prediction unit for predicting mechanical property values including the elongation value and hole expansion rate of the steel plate based on the steel plate 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 the target mechanical property values of the steel plate and determines heat treatment conditions based on the corrected mechanical property values, A heat treatment condition control unit for controlling the steel plate heating temperature at least in the soaking zone among the continuous annealing facilities based on the heat treatment conditions determined by the heat treatment condition calculation unit, A steel plate manufacturing facility further comprising [5] The steel sheet manufacturing facility according to [4], wherein in the heat treatment condition control unit, based on the heat treatment conditions determined by the heat treatment condition calculation unit, the heating temperature of the steel sheet in the soaking zone and the cooling conditions of the steel sheet in the cooling zone are controlled. [6] It is provided with a transformation rate measuring device that measures 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 conveyance direction. In the mechanical property prediction unit, The steel sheet manufacturing facility according to [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.

Advantages 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] <Facility 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 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 (refer to reference sign X in FIG. 1). The looper 4 is a facility for securing the excess 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 that utilizes the 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, following the heating zone 7, by a soaking zone 8 and a cooling zone 10 (a first cooling zone 10A and a second cooling zone 10B) in this order. 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 be configured in a two-stage configuration including the first cooling zone 10A and the second cooling zone 10B as shown in FIG. 1.

[0013] Downstream in the steel sheet conveyance direction X of the cooling zone 10, an annealing post-heat treatment facility 11 configured by a rapid cooling zone 15 and a tempering zone 16 in this order is installed. When the steel sheet S to be manufactured 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 galvanized steel sheet facility), and in the steel sheet conveyance direction X, may have, in order before the rapid cooling zone 15, a hot-dip galvanizing immersion zone 12 and a plating alloying zone 13. When the annealing post-heat treatment facility 11 is a steel sheet plating facility (alloyed hot-dip galvanized steel sheet 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 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.

[0014] In the steel sheet manufacturing facility 1 of the present invention, a temperature measurement device 20 for measuring the temperature of the steel sheet surface is installed at least in the soaking zone 8 and the tempering zone 16, and the temperature change of the steel sheet can be monitored. Further, a transformation rate measurement 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 heating temperature of the steel sheet in the tempering zone 16 measured by the temperature measurement device 20, and a heat treatment condition calculation unit 32 that 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 determines heat treatment conditions based on the corrected mechanical property values. Based on the heat treatment conditions determined by the heat treatment condition calculation unit 32, among the facilities constituting the continuous annealing facility 5, there is a heat treatment condition control unit 33 that controls at least the heating temperature of the steel sheet in the soaking zone 8. 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 the soaking zone 8 described later. Even if the steel sheet manufacturing facility 1 has the preheating zone 6, in the preheating zone 6, the steel sheet S may not be heated, and the steel sheet S such as a thin steel sheet may start to be heated from the next heating zone 7 in the steel sheet conveyance direction X. 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. As a heating method for the preheating zone 6, a method of using the high-temperature exhaust gas generated in the next heating zone 7 can be considered, but it is not particularly limited as long as the target temperature is reached.

[0018] (2) Heating Zone 7 Subsequently, the steel sheet S enters the heating zone 7, and the temperature of the steel sheet is heated to 600 to 700 °C, which is approximately the same as the annealing temperature ( soaking temperature), in order to ensure the maximum holding time in the soaking zone. In the heating zone 7, in addition to the high heating capacity and the small furnace volume, when plating the steel sheet S, the oxidation-reduction reaction on the surface of the steel sheet can be flexibly controlled to ensure the plating property in the subsequent process. 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 becomes easy to control the state 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 gently heated towards the target annealing temperature and held at the annealing temperature. In the soaking zone 8, heating and holding are performed up to a temperature below the A1 transformation point. Thereby, the recrystallization of the α-phase proceeds, and the crystal grains excessively refined by rolling can be coarsened appropriately. Also, the phase fraction of the α-phase and the γ-phase can be adjusted on the outlet side of the soaking zone 8, and the mechanical properties of the product can be controlled by combining with the subsequent cooling. Furthermore, in order to suppress the remaining of the unrecrystallized α-phase, it is preferable to ensure a residence time of 20 to 60 seconds in the above temperature range. The control of the residence time is performed by the heating rate. The heating rate from the heating zone 7 to the arrival of the annealing temperature is preferably 10 °C / s or less, more preferably 5 °C / s or less. As the heating method in the soaking zone 8 of the steel sheet (thin steel sheet) S, it is preferable to adopt the radiation heating (radiant tube heating) method by gas combustion because of high efficiency and heating uniformity. In addition, in order to stabilize the temperature of the steel sheet near the outlet of the soaking zone, by these heating methods, the furnace temperature of the soaking zone is preferably in the range of the target annealing temperature +0 to 50 °C, more preferably controlled in the range of the target annealing temperature +5 to 20 °C. The soaking zone 8 can ensure the residence time in the furnace by reciprocating the steel sheet S between the conveying rolls arranged vertically as shown in Fig. 1. Also, the structure of the furnace itself may be integrated from the inlet to the outlet, or the furnace shell may be separated into an adjustment zone responsible for heating from the recrystallization range to the annealing temperature and a holding zone responsible for holding at the annealing temperature. Thereby, it is also possible to reduce the furnace volume of the holding zone where fine temperature control is required and improve the controllability.

[0020] (4) Cooling zone 10 (first cooling zone 10A, second cooling zone 10B) After being heated to the target annealing temperature in the soaking zone 8, the steel sheet S is conveyed to the cooling zone 10 and cooled. 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 equal to or higher than the martensite transformation start temperature and 550°C or lower, and more preferably within the range of 450°C or higher and 550°C or lower. 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, and more preferably 20 seconds or more and 50 seconds or less after reaching the cooling stop temperature. Cooling methods include 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 passes, water cooling by a water jet, mist cooling in which compressed gas fine water droplets are mixed, etc., but are 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, a first cooling zone (rapid cooling zone) 10A and a second cooling zone (slow cooling / heat retention zone) 10B.

[0021] (5) Plating immersion zone 12 (molten zinc plating immersion zone), plating alloying zone 13, and heat retention zone 14 After performing structure control in the cooling zone 10, the steel sheet S is conveyed to the post-annealing heat treatment facility 11. When performing plating treatment on the steel sheet S, the post-annealing heat treatment facility 11 may be used as a steel sheet plating facility, and the steel sheet S is conveyed to this steel sheet plating facility, and 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 zinc plating method in which the steel sheet is immersed in a zinc plating bath storing molten zinc may be adopted. In addition, in order to control the adhesion amount of zinc 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 with little influence on the plating surface and capable of 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.

[0022] (6) Rapid cooling zone 15 When performing plating treatment on the steel sheet S (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 subjected to plating treatment, 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 compressed gas jets collide, 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 control the cooling stop temperature with high precision while realizing rapid cooling, a combination of mist cooling and gas jet cooling may be used. When plating treatment is performed on the steel sheet S, the steel sheet S that has passed through the plating alloying zone 13 or the heat retention zone 14 and has reached about 350 to 450°C is cooled in the rapid cooling zone 15 to a temperature 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 the other hand, although the upper limit of the cooling rate is not particularly provided, if excessive rapid cooling is performed, the flatness of the steel sheet may be lost 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.

[0023] (7) Tempering zone 16 After creating a martensite structure in the rapid cooling zone 15, tempering is performed 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 retention device (tempering zone heat retention device) 18, and a cooling device (tempering zone cooling device) 19 in the steel sheet 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, in order to obtain the effect of improving toughness by tempering while maintaining the martensite structure. In the heat retention device 18, the temperature of the steel sheet heated in the heating device 17 is maintained, and the retention time in the heat retention 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 an excessive increase in the line length. Thereafter, in the cooling device 19, the steel plate S is cooled to room temperature. With this configuration, 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 and the furnace wall by an electric heater 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.

[0024] (8) Temperature measuring device 20 Among the above-mentioned zones, at least in the soaking zone 8 and the tempering zone 16, a temperature measuring device (thermometer) 20 for measuring the surface temperature of the steel plate is installed. With 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 inside the zone to confirm the intermediate temperature history. Specifically, the installation positions of the above thermometers 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. Also, the installation positions of the above thermometers in each zone are, 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 senses the infrared rays emitted by the steel plate to measure the temperature. 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. Also, since the radiation thermometer is 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 furnace internal conveying roll and the steel plate S may be adopted.

[0025] (9) Transformation rate measuring device 21 It is preferable that at least one transformation rate measuring device 21 for measuring the austenite fraction of the steel sheet S is provided from the outlet side of the soaking zone 8 to the outlet side of the tempering zone 16. This is because, by predicting the mechanical property values (for example, elongation value, hole expansion rate) of the steel sheet S based on the measured austenite fraction, the prediction accuracy of the mechanical property values of the steel sheet S is improved compared to predicting the mechanical property values such as the elongation value and hole expansion rate of the steel sheet S only from the heating temperature of the steel sheet in the tempering zone 16. With this transformation rate measuring device 21, information on the fractions (transformation rates) of the α-phase and γ-phase of the steel sheet 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, and enables stable acquisition of desired mechanical properties. The method for measuring the transformation rate is not particularly limited, but the transformation rate measuring device 21 may be a magnetic detector, that is, a device for measuring the magnetic transformation rate of the steel sheet S (steel strip). It can be configured 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, and can measure the austenite fraction. 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 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.

[0026] (Mechanical property values (elongation value, hole expansion rate)) As the elongation value among the mechanical property values of the steel sheet S of the present invention, it is determined based on the test method of JIS Z2241 (2011), for example. The sample for measuring the elongation value is preferably collected from the steel sheet cut by the cut-off immediately after the treatment 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 hole expansion rate among the mechanical property values of the steel sheet S of the present invention, it is determined based on the test method of JIS Z2256 (2010), for example. Samples for measuring the hole expansion ratio are preferably taken from steel plates cut just before coiler winding. Also, for each coil, samples are taken from the head and tail ends of the coil during operation.

[0027] <Control method> FIG. 2 is a flowchart for explaining the control method of the steel plate manufacturing facility 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 of FIG. 2.

[0028] (1) Mechanical property prediction unit 31, mechanical property prediction step In the steel plate manufacturing facility 1, a new steel plate S is manufactured (step S1), and at the joint portion of the steel plate 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 plate manufacturing facility 1 continues to manufacture the steel plate S. The determination in this step S2 may be performed by a control unit (not shown) in the management device 30 of the steel plate manufacturing facility 1, or may be performed by another control unit (not shown) included in the steel plate manufacturing facility 1. 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 plate S (succeeding material) based on the operating conditions (steel plate heating temperature) in the tempering zone (reheating zone) 16. Examples of the mechanical property values include elongation values and hole expansion ratios. As operation conditions input to the mechanical property prediction unit 31, there are also operation conditions in the equipment downstream of the soaking zone 8. Based on these operation conditions, the mechanical properties when the trailing 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 operation conditions and performing the prediction of the mechanical property values based on this austenite fraction and the operation conditions, the prediction accuracy of the mechanical property values is improved. Since only temperature measurement indirectly predicts the microstructure, by directly measuring the austenite fraction, the prediction accuracy of the mechanical properties after the heat treatment process is improved. As a method for predicting mechanical properties performed by the mechanical property prediction unit 31, regarding the elongation value and the hole expansion rate, for example, a method using 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 operation 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 heat treatment of each steel plate can be used. As this procedure, heat treatment simulating an actual machine is performed 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 elongation value prediction database, or predict the elongation value of the steel plate S based on the data.

[0029] (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 plate S (trailing material) predicted by the mechanical property prediction unit 31 in the mechanical property prediction step S3 are corrected to the target mechanical property values, and the heat treatment conditions in the continuous annealing facility 5 are calculated based on the corrected mechanical property values. The correction mentioned here refers to changing the mechanical property values for calculating the heat treatment conditions from the predicted mechanical property values to the preset target mechanical property values. At this time, when the heat treatment condition calculation unit 32 determines that the target mechanical property values can be corrected only by changing the steel plate heating conditions in the soaking zone 8 (step S5), it calculates and determines the conditions of the steel plate heating temperature in the soaking zone 8 (step S6a). On the other hand, when it is determined that the target mechanical property values cannot be corrected only by changing the steel plate heating temperature in the soaking zone 8 (step S5), the heat treatment condition calculation device 32 calculates the steel plate cooling conditions in the cooling zone 10 in addition to the heating temperature in the soaking zone 8 (step S6b). As a method for determining the heat treatment conditions (the steel plate heating temperature in the soaking zone 8 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 method using 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 the 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.

[0030] (3) Heat treatment condition control unit 33, heat treatment condition control steps In the heat treatment condition control step S7, the heat treatment condition control unit 33 controls the steel plate heating temperature in the soaking zone 8 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 heating temperature of the steel plate in the soaking zone 9 but also the cooling conditions of the steel plate in the cooling zone 10 are controlled, so that the mechanical properties (elongation value, hole expansion rate) of the steel plate S (rear pass zone) can be controlled more accurately.

[0031] (4) Transition to the steady state The management device 30 determines whether the heat treatment conditions determined based on the heating temperature of the steel plate 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 mechanical property prediction step S3, the heat treatment condition calculation step S4, and the heat treatment condition control step S7 described above, and for the subsequent material, adjusts the heat treatment conditions other than the above target heat treatment conditions (steady heat treatment conditions) (transition to the 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).

[0032] By performing the control described with reference to FIG. 2, for example, even when there is a control delay and the operating conditions in the tempering zone 16 are different from the target tempering conditions of the subsequent material, and there is a concern that 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 the target mechanical property values are calculated by the heat treatment condition calculation unit 32, and the heat treatment conditions in the soaking zone 8 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

[0033] 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 successively manufactured. Five pieces of each strength were passed through continuously, and a total of 150 pieces were manufactured in 10 sets with 15 pieces of three types as one set for each example. The sheet thickness was in the range of 1.0 to 2.0 mm, and the manufacturing order was determined so that the sheet thickness was almost constant within a set, and sets with gradually changing sheet 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 a continuous casting machine as materials. More specifically, although within the manufacturing management range, the chemical compositions of the slabs varied and the transformation behaviors were not uniform.

[0034] Each slab was subjected to hot rolling, pickling, annealing if necessary, and cold rolling in a conventional manner, and 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 positions between products of different strengths, analytical samples were taken every 10 m in the range of 100 m before and after the connection position to examine the region where the mechanical properties did not meet the shipping standards, and the ratio of the shippable length 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, 980 MPa, 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.

[0035] 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-fired heating) was kept in the range of 600 - 700 °C, and the annealing temperature was set 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 taken from the final product to investigate the variation in mechanical properties.

[0036] Table 1 shows the production results under each production condition.

[0037]

Table 1

[0038] Comparative Example 1 is an example in which a product was 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 Reference Document 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 an induction heating device was provided on the outlet side of the annealing furnace (after the cooling zone) of Comparative Example 2. 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".

[0039] As shown in Table 1, in Comparative Example 1, the TS within products of the same strength had a large variation, and there were some 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 "-".

[0040] 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. Moreover, the variation in mechanical properties was large even within the coil, and in the connection part, a wide area that did not meet the material test criteria occurred, resulting in a low yield in the connection part.

[0041] 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 some products did not meet the acceptance criteria. Also, the yield in the connection part was not sufficiently improved.

[0042] For these comparative examples, Example 1 is the result of manufacturing products from the above-mentioned materials using the control method of the steel plate manufacturing facility in the present invention. Example 3 is an example in which a transformation rate 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, in 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 products in 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 rate measuring device.

Explanation of symbols

[0043] S Steel plate X Steel plate conveying direction 1 Steel plate manufacturing facility 2 Payoff reel 3 Welder 4 Looper 5 Continuous annealing facility 6 Preheating zone 7 Heating zone 8 Soaking 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 Retention 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 continuous annealing facility for steel sheets having, in the steel sheet conveying direction, a heating zone, a soaking zone, and a cooling zone in this order, An after-annealing heat treatment facility having, downstream of the cooling zone in the steel sheet conveying direction, at least a rapid cooling zone and a tempering zone in this order, A temperature measuring device for the steel sheet installed in at least the soaking 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 soaking 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 soaking 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 conveying 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 conveying direction, a heating zone, a soaking zone, and a cooling zone in this order; An after-annealing heat treatment facility having, downstream of the cooling zone in the steel sheet conveying direction, at least a rapid cooling zone and a tempering zone in this order; A temperature measuring device for the steel sheet installed in at least the soaking zone and the tempering zone; Characterized in that it comprises a mechanical property prediction unit for 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 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 soaking 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, based on the heat treatment conditions determined by the heat treatment condition calculation unit, the heating temperature of the steel sheet in the soaking zone and the steel sheet cooling conditions in the cooling zone are controlled.

6. Comprising a transformation rate measuring device that measures 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 conveyance 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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