Continuous annealing equipment, continuous annealing method, cold-rolled steel sheet manufacturing method, and plated steel sheet manufacturing method

Abstract

Provided are continuous annealing equipment, a continuous annealing method, a cold-rolled steel sheet manufacturing method, and a plated steel sheet manufacturing method wherein the occurrence of quality variation in the sheet width direction can be minimized. This continuous annealing equipment is for a steel sheet and is configured comprising a heating zone (6), a soaking zone (7), and a cooling zone (8), in this order, said continuous annealing equipment comprising: an induction heating device (9) that is provided between the soaking zone and the cooling zone; a cooling device (13) that uses a gas to cool an end section of the steel sheet at the entrance to the induction heating device; and a control device that controls the operating conditions of the induction heating device and the cooling device on the basis of the phase fractions of the steel sheet downstream of the induction heating device.

Description

Continuous annealing equipment, continuous annealing method, manufacturing method of cold-rolled steel sheet, and manufacturing method of plated steel sheet

[0001] The present disclosure relates to continuous annealing equipment, a continuous annealing method, a method for manufacturing a cold-rolled steel sheet, and a method for manufacturing a plated steel sheet.The present disclosure particularly relates to continuous annealing equipment, a continuous annealing method, a method for manufacturing a cold-rolled steel sheet, and a method for manufacturing a plated steel sheet for manufacturing a high-strength steel sheet used for automotive structural materials, etc.

[0002] In the production of automotive steel sheets, continuously cast slabs are extensively processed by hot rolling and cold rolling until they reach the final thickness. The subsequent annealing process restores the cold-worked structure, recrystallizes, and grows the grains, and further controls the transformed structure to adjust the balance between strength and workability.

[0003] In recent years, a continuous annealing furnace has been generally used for annealing, in which a continuous strip of steel sheet is continuously heated, soaked, and cooled while being transported. After the cooling, hot-dip galvanizing and overaging treatments are performed depending on the application of the steel sheet.

[0004] In the production of thin steel sheets for automobiles, temperature control of the steel sheet is extremely important to obtain the desired material properties. For example, a common heating method in annealing furnaces is a radiant tube burner, which uses gas combustion to heat a metal tube (radiant tube), and then indirectly heats the steel sheet with the radiant heat.

[0005] Radiant tube furnaces heat steel sheets using radiant heat from the radiant tubes and furnace walls, resulting in a very large heat source volume and large thermal inertia. This makes it difficult to quickly respond to changes in the set temperature. Furthermore, the temperature rise rate of the steel sheet is slow toward the end of heating, and a certain soaking time is required for structural control. This increases the required furnace length and further increases thermal inertia, further delaying the response to the target temperature. As a result, during the continuous processing of coils, the temperature of some steel sheets may not fall within the specified annealing temperature range, resulting in problems such as reduced yield due to variations in mechanical properties and reduced productivity due to changes in line speed for temperature control.

[0006] To address the above-mentioned problems, induction heating devices are sometimes installed to improve productivity and suppress material variations. However, when heating steel sheets using induction heating devices, there is a problem of overheating at the edges of the steel sheets. To suppress overheating, for example, Patent Document 1 discloses a technique in which, when a solenoid-type induction heating device is used, a magnetic flux density reduction means is provided to reduce magnetic flux concentrating at the edges of the steel sheets. For example, Patent Document 2 discloses a technique in which an induction heating device is provided with a cooling means that forcibly cools the edges of the steel sheets. For example, Patent Document 3 discloses a technique in which, when a transverse-type induction heating device is used, the temperature of the steel sheets is uniformed, taking into account the temperature drop specific to edge masks.

[0007] Japanese Patent Laid-Open No. 1-232685 Japanese Patent Laid-Open No. 2001-6860 Japanese Patent Laid-Open No. 2019-179733

[0008] The techniques of Patent Documents 1 to 3 are aimed at uniforming the temperature of the steel sheet after annealing. Although the techniques of Patent Documents 1 to 3 may ultimately suppress the influence of changes in the material properties of the steel sheet at its edges due to overheating, there is a demand for further suppression of variations in the material properties of the steel sheet.

[0009] The technique of Patent Document 1 suppresses overheating of the end portions of the steel sheet, but when applied to an annealing furnace, it does not homogenize the material, including the material variations in the sheet width direction that have already occurred due to the influence of the thermal history or processing history in the hot rolling process or cold rolling process prior to the annealing furnace.

[0010] Furthermore, although the techniques of Patent Documents 2 and 3 are intended to be applied to an annealing furnace, they do not homogenize the material, including the material variations in the sheet width direction that have already occurred in processes prior to the annealing furnace.

[0011] In view of the above problems, the present disclosure aims to provide continuous annealing equipment, a continuous annealing method, a method for manufacturing a cold-rolled steel sheet, and a method for manufacturing a plated steel sheet, which are capable of suppressing the occurrence of material variation in the sheet width direction.

[0012] (1) A continuous annealing facility according to one embodiment of the present disclosure is a continuous annealing facility for a steel sheet configured to include a heating zone, a soaking zone, and a cooling zone in this order, and includes: an induction heating device provided between the soaking zone and the cooling zone; a cooling device that cools an end of the steel sheet with gas at an inlet of the induction heating device; and a control device that controls operating conditions of the induction heating device and the cooling device based on a phase fraction of the steel sheet downstream of the induction heating device.

[0013] (2) As one embodiment of the present disclosure, in (1), the phase fraction is measured using a transformation rate meter installed downstream of the induction heating device.

[0014] (3) A continuous annealing method according to one embodiment of the present disclosure is a continuous annealing method carried out in continuous annealing equipment for steel sheets, which is configured to include a heating zone, a soaking zone, and a cooling zone in this order, and includes the steps of: performing rapid heating by an induction heating device provided between the soaking zone and the cooling zone; cooling an end of the steel sheet at an inlet of the induction heating device by a cooling device; and controlling operating conditions of the induction heating device and the cooling device based on the phase fraction of the steel sheet downstream of the induction heating device.

[0015] (4) As one embodiment of the present disclosure, in (3), the induction heating device heats the steel plate at 10°C / s or more and 200°C / s or less.

[0016] (5) As an embodiment of the present disclosure, in (3) or (4), the phase fraction is measured using a transformation rate meter installed downstream of the induction heating device.

[0017] (6) A method for producing a cold-rolled steel sheet according to an embodiment of the present disclosure includes annealing the steel sheet, which is a cold-rolled steel sheet, by any one of the continuous annealing methods (3) to (5).

[0018] (7) A method for producing a plated steel sheet according to an embodiment of the present disclosure includes a step of performing a plating treatment on the surface of the steel sheet annealed by the method for producing a cold-rolled steel sheet according to (6).

[0019] (8) As one embodiment of the present disclosure, in (7), the plating treatment is an electrogalvanizing treatment, a hot-dip galvanizing treatment, or a hot-dip galvannealing treatment.

[0020] According to the present disclosure, it is possible to provide continuous annealing equipment, a continuous annealing method, a method for manufacturing a cold-rolled steel sheet, and a method for manufacturing a plated steel sheet that can suppress the occurrence of material variations in the sheet width direction.

[0021] Fig. 1 is a schematic diagram showing a general configuration of a continuous annealing facility according to an embodiment of the present disclosure. Fig. 2 is a schematic diagram showing another configuration of a continuous annealing facility according to an embodiment of the present disclosure. Fig. 3 is a schematic diagram showing the arrangement relationship between a cooling device and an induction heating device. Fig. 4 is a flowchart showing an example of a continuous annealing method.

[0022] Hereinafter, a continuous annealing facility, a continuous annealing method, a method for manufacturing a cold-rolled steel sheet, and a method for manufacturing a plated steel sheet according to an embodiment of the present disclosure will be described with reference to the drawings. In each drawing, the same or corresponding parts are designated by the same reference numerals. In the following description of the embodiment, the description of the same or corresponding parts will be omitted or simplified as appropriate.

[0023] <Equipment Configuration> FIG. 1 shows a portion of a hot-dip galvanizing process equipped with continuous annealing equipment according to this embodiment. In this embodiment, the steel material produced using the hot-dip galvanizing process is a thin steel sheet. The steel material produced is a cold-rolled steel sheet. The arrow in FIG. 1 indicates the line traveling direction. Hereinafter, the upstream side in this traveling direction may be referred to as the "front" and the downstream side as the "rear." The continuous annealing equipment includes a payoff reel 1, a welding machine 2, an electrolytic cleaning device 3, an entry looper 4, a preheating zone 5, a heating zone 6, a soaking zone 7, and a cooling zone 8. The continuous annealing equipment also includes an induction heating device 9 (hereinafter sometimes referred to as "IH"), a transformation rate meter 10, and a cooling device 13. The continuous annealing equipment includes at least one induction heating device 9. In this embodiment, the continuous annealing equipment includes at least one transformation rate meter 10. However, the continuous annealing equipment is not limited to a configuration including the transformation rate meter 10, as long as it can grasp the phase fraction after annealing in the induction heating device 9. For example, the continuous annealing equipment may be configured to predict the transformation rate based on information from an installed thermometer, instead of the transformation rate meter 10. The continuous annealing equipment also includes at least one cooling device 13. In this embodiment, the cooling zone 8 includes a first cooling zone 8A and a second cooling zone 8B. The continuous annealing equipment may further include a galvanizing tank (zinc pot 11) in which the thin steel sheet cooled to a predetermined temperature is immersed, an alloying zone, a holding zone, a final cooling zone, a temper rolling facility, an outlet looper, a tension reel, and the like.

[0024] A steel sheet wound into a coil in a previous process is unwound by a payoff reel 1. The unwound steel sheet passes through a preheating zone 5 and then enters a continuous annealing furnace. The continuous annealing equipment is configured to include, in this order, a heating zone 6, a soaking zone 7, and a cooling zone 8 following the preheating zone 5. In the continuous annealing equipment, an induction heating device 9 is provided between the soaking zone 7 and the cooling zone 8. The continuous annealing equipment also includes a transformation rate meter 10 downstream of the induction heating device 9. In the example of FIG. 1 , the transformation rate meter 10 is provided between the induction heating device 9 and the cooling zone 8, but the installation position is not limited as long as it is downstream of the outlet of the induction heating device 9.

[0025] As shown in the example of Fig. 2, the transformation rate meter 10 can be installed immediately after the induction heating device 9. Here, the transformation rate meter 10 is an example of a measuring device that measures the austenite fraction (γ phase fraction) of a steel sheet. In order to grasp the austenite fraction, a thermometer may be installed in each annealing zone in the sheet width direction, and the results predicted from the steel sheet dimensions, actual composition, etc. may be used. Here, the sheet width direction refers to the width direction of the steel sheet.

[0026] Furthermore, the continuous annealing equipment is configured to include a mechanism for adjusting the output of the induction heating device 9 and the output of the cooling device 13 using the measurement results of the γ phase fraction by the transformation rate meter 10. The continuous annealing equipment may be configured to change other annealing equipment conditions, such as the furnace temperature of the soaking zone 7, as necessary. The continuous annealing equipment according to this embodiment is not limited to the configuration shown in FIG. 1 and may be modified depending on the target quality of the steel sheet to be manufactured. For example, a zinc pot 11 and an alloying zone may be provided following the continuous annealing furnace, or the zinc pot 11 may be omitted. Furthermore, the continuous annealing equipment may be partially divided. For example, the soaking zone 7 may be divided, and the atmosphere or temperature of each divided soaking zone 7 may be independently controlled.

[0027] (Preheating Zone) The thin steel plate discharged from the payoff reel 1 at a temperature between room temperature and about 100°C first enters the preheating zone 5 and is heated to about 200°C. In this embodiment, the preheating zone 5 uses the high-temperature exhaust gas generated in the heating zone 6.

[0028] (Heating Zone) Next, the steel sheet enters the heating zone 6, where it is heated to a temperature of approximately 600 to 700°C. In this embodiment, a direct-fired heating furnace is used in the heating zone 6 in order to raise the temperature to a certain level in a short time and to control the surface condition. A direct-fired heating furnace has high heating capacity and allows the furnace volume to be small, and also allows for flexible control of the oxidation-reduction reaction on the steel sheet surface, taking into account the subsequent plating process.

[0029] (Soaking Zone) The main role of the soaking zone 7 is to promote the recrystallization of the α phase. １It is acceptable for there to be a portion where the temperature passes through the transformation point. As a heating method for the soaking zone 7, it is desirable to adopt a radiant heating method (radiant tube heating) using gas combustion because of its high efficiency and high heating uniformity. The soaking zone 7 may be configured as a large single furnace shell. Alternatively, the soaking zone 7 may have two or more compartments, each of which may be separated by a heat insulating wall or the like, and may be configured to maintain and heat within a target temperature range determined for each compartment. In this embodiment, the soaking zone 7 is configured to heat the steel sheet until the temperature reaches A １ The temperature is held or slowly heated to a temperature range lower than the transformation point (approximately 600 to 730°C), ensuring a residence time in the recrystallization temperature range and suppressing the retention of unrecrystallized α phase. １ The transformation point is the temperature at which austenite transformation occurs, and the set furnace temperature is 730°C, for example. １ At temperatures above the transformation point, the austenite phase begins to form.

[0030] Here, if the temperature of the steel sheet in the soaking zone 7 is too low, the recrystallization of the α phase does not proceed, and sufficient workability cannot be obtained. １ If the temperature is above the transformation point, the α-recrystallized grains become coarse and the strength is insufficient. １ The transformation point may vary slightly depending on the composition of the steel sheet. Therefore, it is preferable to predict the recrystallization temperature range in advance by measurement, calculation, or simulation, and set it taking into account the error range during annealing.

[0031] The continuous annealing equipment according to this embodiment includes a soaking zone 7 using a radiant tube furnace. However, the equipment may be configured without such a soaking zone 7 as long as it has the function of ensuring a heat retention time sufficient to promote α-phase recrystallization. For example, the soaking zone 7 is not limited to one including a radiant tube furnace. The continuous annealing equipment may not include the soaking zone 7, and the heating zone 6 may have the function of ensuring a heat retention time sufficient to promote α-phase recrystallization. The continuous annealing equipment may also include a heat retention device instead of the soaking zone 7. When the continuous annealing equipment does not include the soaking zone 7, the induction heating device 9 may be provided between the part that functions as the soaking zone 7 (in the above example, the part that ensures the heat retention time of the heating zone 6 or the heat retention device) and the cooling zone 8.

[0032] (Induction Heating Device) Next, the induction heating device 9 is １ Above transformation point and A ３ The output is adjusted so that the steel sheet is rapidly heated to a temperature range (approximately 750 to 900°C) lower than the transformation point. ３ The transformation point is the upper limit temperature at which the γ phase fraction can be controlled. This process is carried out in a short time, uniformly across the entire steel sheet, so that the same γ phase fraction can be obtained at any point on the steel sheet. １ The purpose is to raise the temperature to a temperature equal to or higher than the transformation point. By adopting the induction heating device 9, it is possible to contribute to the miniaturization of the entire equipment. １ Transformation point and A ３ The transformation point may vary slightly depending on the composition of the steel sheet, and therefore, it is preferable to predict the suitable temperature range in advance by measurement, calculation, or simulation, and set it taking into consideration the error range during annealing.

[0033] In the case of DP steel, which is one of the targets of the continuous annealing equipment according to this embodiment, the temperature reached by the induction heating device 9 has a significant impact on the mechanical properties of the final product. Therefore, the heating method of the induction heating device 9 must be able to respond quickly to temperature control commands. Furthermore, when heating in this temperature range, the Curie point at which the magnetic properties of the steel sheet change is exceeded, so it is desirable that the induction heating device 9 be of the transverse type.

[0034] Furthermore, since the soaking zone 7 maintains the α-phase recrystallization region, if heating takes too long, the α-grains will become coarse. Therefore, an induction heating device 9 capable of rapid heating is desirable. The induction heating device 9 may increase the temperature at a rate of 10°C / s or more and 200°C / s or less. A heating rate of less than 10°C / s will result in coarsening of the α-grains, while a heating rate of more than 200°C / s will result in localized high-temperature areas in the sheet width direction, making it difficult to maintain uniformity. It is more preferable that the induction heating device 9 increase the temperature at a rate of 20°C / s or more and 100°C / s or less. A heating rate of 20°C / s or more allows for a shorter line length, while a heating rate of 100°C / s or less further reduces the risk of buckling deformation of the steel sheet due to thermal stress.

[0035] When the steel sheet is rapidly heated to the target annealing temperature by the induction heating device 9, the transformation from the α phase to the γ phase may not reach equilibrium immediately after heating. However, if the steel sheet is held at a temperature close to the target annealing temperature for too long, the transformation from the α phase to the γ phase progresses more than necessary, resulting in hardening and preventing the required elongation from being obtained in the final product. Therefore, after the target annealing temperature is reached, it is most desirable to enter the cooling zone 8 as quickly as possible with a holding time of 0 seconds, and it is desirable to start cooling within 5 seconds. To avoid a non-equilibrium state, even if the steel sheet is held, cooling is started within 10 seconds at the latest. In other words, the steel sheet starts cooling in the cooling zone 8 within 10 seconds after heating by the induction heating device 9 is completed.

[0036] In order to satisfy these conditions, the induction heating device 9 may be provided between the soaking zone 7 and the cooling zone 8. For example, if the induction heating device 9 is provided at the connection between the soaking zone 7 and the cooling zone 8, it can be considered as an additional induction heating device 9 for an existing furnace.

[0037] Furthermore, when a divided soaking zone 7 is used, two or more induction heating devices 9 may be installed. For example, the soaking zone 7 may be configured to be divided into two zones, with the upstream zone performing holding in the recrystallization zone and the downstream zone performing holding in a high temperature zone to promote the reduction reaction on the steel sheet surface. Since the optimal temperature ranges differ for each zone, a first induction heating device 9 that rapidly heats the steel sheet from the low temperature zone to the temperature of the high temperature zone, and a second induction heating device 9 that adjusts the temperature for material quality control immediately before entering the cooling zone 8 may be installed.

[0038] (Cooling device) When a steel sheet is heated by an induction heating device 9, eddy currents generated in the steel sheet tend to concentrate at both ends (edges) in the sheet width direction. For this reason, it is known that both ends in the sheet width direction become higher in temperature than the central portion. In order to make the temperature of the steel sheet uniform in the sheet width direction after heating, a cooling device 13 that pre-cools both ends of the steel sheet is installed on the inlet side of the induction heating device 9, as shown in Figure 3.

[0039] The cooling device 13 is preferably a device that cools by ejecting gas. In this embodiment, the cooling device 13 cools the edge of the steel plate with gas. The cooling device 13 is equipped with nozzles, and the nozzles may be attached to the front and back sides of the steel plate so as to face each other across the steel plate, so that gas is ejected onto both sides (front and back) of the edge of the steel plate.

[0040] The range in which overheating occurs is a range of approximately 200 to 300 mm inward from each edge of the steel plate in the plate width direction. Therefore, as long as the nozzles can be positioned so as to specifically cool this range, the nozzle attachment is not limited to a specific form. Here, it is more preferable to determine the cooling range so that the material variation is within a target range across the entire plate width direction, depending on the transformation rate distribution in the plate width direction measured using the transformation rate meter 10. It is preferable to install multiple cooling devices 13 in the plate width direction so that the cooling range can be changed.

[0041] The nozzle of the cooling device 13 may be a slit nozzle with a slit extending in the width direction of the sheet, or a nozzle with small holes drilled in a nozzle header extending in the width direction of the sheet. The thickness of the slit can be set to any width, for example, between 1 and 5 mm. The diameter of the small holes can be set to any diameter, for example, between 1 and 20 mm. When the cooling amount is insufficient with a single nozzle, multiple nozzles can be installed. The cooling gas is a non-oxidizing gas (N ２ , H ２ In this case, the cooling gas is preferably not a high-temperature gas inside the furnace but a gas drawn in from outside the furnace.

[0042] The cooling capacity of the cooling device 13 may be determined based on the speed at which the steel plate is passed, the dimensions of the steel plate, and the specifications of the induction heating device 9. The cooling device 13 has a heat transfer coefficient of 50 to 500 W / (m ２ It is desirable to have a cooling performance that can realize a heat transfer coefficient of 500 W / (m ２ If the temperature exceeds K, the shape of the steel plate may be distorted, which may cause problems during transportation.

[0043] If the cooling device 13 and the induction heating device 9 are spaced apart, the steel sheet will be heated by the ambient atmosphere before reaching the induction heating device 9. As a result, the temperature distribution on the outlet side of the induction heating device 9 will not be uniform, causing variations in the material quality across the width. The heat transfer coefficient of heating due to forced convection in the furnace atmosphere is approximately 20 W / (m ２ Assuming a strip passing speed of 100 mpm and a strip thickness of 1 mm, the temperature rise at the exit side of the soaking zone is about 5°C at a distance of 4 m and about 10°C at a distance of 8 m. From the above, in order to equalize the temperature variation in the width direction, the distance from the exit side of the cooling device 13 to the entry side of the induction heating device 9 is preferably within 8 m, and more preferably within 4 m.

[0044] (Transformation Rate Meter) A transformation rate meter 10 for measuring the phase fraction of the steel sheet is disposed downstream of the induction heating device 9, and measures the phase fractions (transformation rates) of the α and γ phases of the steel sheet after the target annealing temperature is reached. This transformation rate meter 10 is provided for the purpose of adjusting the operating conditions of the induction heating device 9, the cooling device 13, and the like based on the measured γ phase fraction. Such adjustments enable stable mechanical properties of the steel sheet to be obtained. Here, the installation location of the transformation rate meter 10 is not particularly limited as long as it is downstream of the induction heating device 9. However, from the viewpoint of feeding back measured values ​​and using them to control the induction heating device 9 and the like, it is desirable to install it as close to the induction heating device 9 as possible. From this viewpoint, it is preferable to install it upstream of the outlet of the cooling zone. However, the heat resistance of the transformation rate meter 10 needs to be taken into consideration when installing it. For faster feedback, it is more preferable to install the transformation rate meter 10 between the outlet of the induction heating device 9 and the entrance of the cooling zone 8.

[0045] Here, the method for measuring the transformation rate is not particularly limited. For example, the austenite fraction may be measured using a magnetic detector, i.e., a magnetic transformation rate measurement device consisting of a drive coil that generates a magnetic field and a detection coil that measures the magnetic field that has passed through the steel strip. Specifically, the device described in JP 2019-7907 A can be used. However, in the case of a magnetic transformation rate measurement device, if the transformation rate meter 10 is installed near the induction heating device 9, the measurement accuracy may be reduced due to the influence of the magnetic field. As a method that is not affected by the magnetic field, a device that applies X-ray diffraction can be used. Due to differences in crystal structure, the γ phase and the α phase each produce diffraction peaks at unique angles when X-rays are irradiated onto a steel sheet. The γ phase fraction can be quantified based on the intensity of this diffraction peak. Commercially available devices include, for example, X-CAP manufactured by SMS.

[0046] The transformation ratio is preferably measured so as to obtain the phase fraction distribution in the sheet width direction. When it is difficult to measure the phase fraction distribution in the sheet width direction, it is sufficient to measure at least three locations, namely, the sheet width center portion and both sheet width end portions. The sheet width center portion is the central portion of the steel sheet in the sheet width direction, and the center of the steel sheet may be measured. The sheet width end portions are portions close to each of both edges in the sheet width direction. The measurement position at the sheet width end portions is not limited to a specific position, but it is necessary to measure at least one location within a range of 25 to 100 mm inward from the edge of the end portion of the steel sheet.

[0047] Here, as described above, the method for determining the transformation rate is not limited to measurement using the transformation rate meter 10. As another method, a predicted value of the phase fraction may be used by calculation using a machine learning model or a physical model. In this case, it is desirable to appropriately install a thermometer inside the annealing furnace to improve the accuracy of the prediction model.

[0048] (Cooling Zone) The cooling zone 8 is a facility for cooling the steel sheet to a predetermined temperature, and gas jet cooling, roll cooling, water cooling (water quenching), or the like is used as a cooling means. As in the present embodiment, the cooling zone 8 may be divided into a plurality of zones, such as a first cooling zone 8A and a second cooling zone 8B, and the thermal history of the steel sheet during cooling may be controlled by combining different cooling means or by changing the cooling conditions of the same type of cooling means. Furthermore, when the cooling zone is divided, a transformation rate meter 10 may be installed in the divided section.

[0049] (Hot-dip galvanizing bath) A hot-dip galvanizing bath is provided following the cooling zone 8, and hot-dip galvanizing can be applied to the surface of the steel sheet discharged from the cooling zone 8. Hot-dip galvanizing may be performed according to a conventional method, and a snout, a bath roll, etc. may be provided as necessary.

[0050] (Alloying equipment) An alloying treatment equipment may be provided following the hot-dip galvanizing bath. In the alloying treatment equipment, the steel sheet is heated and subjected to alloying treatment. The alloying treatment may be carried out according to a conventional method.

[0051] (Other Equipment) Following the alloying equipment, a holding zone, a final cooling zone, temper rolling equipment, a straightener, an outlet looper, a tension reel, and the like may be further provided in order to improve the quality of the final product and to ensure stable and efficient production. These pieces of equipment may be provided and used according to the quality required of the product, and are not particularly limited. Furthermore, if a shape disorder occurs, straightening may be performed by passing the steel sheet through temper rolling equipment and a straightener. Temper rolling and straightening may be performed under conditions that allow the shape to be straightened without affecting the mechanical properties of the steel sheet.

[0052] <Operation Method (Control Method)> In this embodiment, a "transformation ratio control model" is constructed by determining in advance the optimum phase fraction range for each product, and the output of the induction heating device 9 or the output of the cooling device 13 is controlled so that the phase fraction measurement results obtained by the transformation ratio meter 10 fall within the target range. In this embodiment, other annealing equipment conditions, such as the furnace temperature of the soaking zone 7, are also controlled as needed. This makes it possible to further suppress material variations in the sheet width direction, compared to a method of simply uniforming the steel sheet temperature during annealing in the sheet width direction, and to manufacture products with very stable mechanical properties.

[0053] 4 is a flowchart showing an example of a continuous annealing method performed in the continuous annealing facility according to the present embodiment. This continuous annealing method may be performed by a control device attached to the continuous annealing facility, such as a process computer that accumulates and manages operational condition records.

[0054] The phase fraction of the steel sheet that has passed through the induction heating device 9 is acquired as "actual transformation rate information." The phase fraction may be a measured value or a predicted value. Furthermore, information on the steel sheet dimensions, such as the chemical composition, thickness, and width, obtained when the material is cast is acquired as "material information." Other manufacturing conditions for the passing process may also be acquired as material information. Information on the steel sheet conveying speed during annealing, the set furnace temperature, the furnace atmosphere gas, and the cooling conditions is acquired as "operating conditions of the continuous annealing equipment." Based on this information, a "material quality prediction model" is used to predict the mechanical properties (mechanical characteristics) of the final product. The material quality prediction model may be a physical model obtained by offline experiments or numerical analysis. Furthermore, the material quality prediction model may be a machine learning model generated by machine learning using accumulated manufacturing results as learning data.

[0055] Next, it is determined whether the predicted mechanical properties derived from the material prediction model are within the target range of mechanical properties acceptable for the product. The target range of mechanical properties is provided by a host computer. The host computer is a computer that provides manufacturing information, etc. to a process computer that oversees the operation of the continuous annealing equipment. If the properties are within the target range ("Mechanical Properties Within Target Range" in Figure 4 is Yes), the continuous annealing equipment continues operation under the current conditions (i.e., the same conditions). If the properties are outside the target range ("Mechanical Properties Within Target Range" in Figure 4 is No), the continuous annealing equipment changes the operating conditions. When changing the operating conditions, a "transformation rate control model" is used to determine the phase fraction adjustment amount required to bring the material within the target range from the material prediction model, and to determine the steel sheet temperature control amount to achieve the transformation rate adjustment amount. The "transformation rate control model" may be a physical model obtained by offline laboratory experiments or numerical analysis, or a machine learning model obtained from accumulated manufacturing experience. Here, the gamma phase is suitable as the metal phase to be adjusted.

[0056] When the continuous annealing facility determines that the required steel sheet temperature control amount is within the controllable range of the cooling device 13 on the inlet side of the induction heating device 9 ("Within cooling device control range" in FIG. 4 is Yes), it changes the cooling amount by only changing the output of the cooling device 13 ("Change cooling amount of cooling device" in FIG. 4). When the continuous annealing facility determines that the required steel sheet temperature control amount is not within the controllable range of the cooling device 13 ("Within cooling device control range" in FIG. 4 is No), it also changes the conditions of the induction heating device 9.

[0057] When the continuous annealing facility determines that the required steel sheet temperature control amount is within the range controllable by the induction heating device 9 ("Within IH output control range" in FIG. 4 is Yes), it changes the output of the induction heating device 9. When the continuous annealing facility determines that the required steel sheet temperature control amount is not within the range controllable by the induction heating device 9 ("Within IH output control range" in FIG. 4 is No), it also changes the conditions of the cooling device 13 and the equipment other than the induction heating device 9 ("Change other annealing equipment conditions" in FIG. 4). Here, the other annealing equipment refers to the soaking zone 7 or the cooling zone 8, etc. Although the other annealing equipment has lower responsiveness to changes in operating conditions than the induction heating device 9, by constantly controlling it in combination with the induction heating device 9 via the above control flow even while the conditions are changing, it is possible to minimize the amount by which the mechanical properties of the product deviate from the target.

[0058] Regarding the above-mentioned cooling amount, when the steel sheet temperature at the completion of annealing predicted based on the transformation rate distribution in the sheet width direction varies across the sheet width, the cooling device 13 preliminarily imparts a temperature difference between the sheet width end portion and the sheet width center portion to eliminate this difference. The cooling amount for imparting the temperature difference is determined by the following method.

[0059] For example, the temperature T １ When the steel plate passes through the induction heating device 9, the temperature at the center of the plate width increases to T ２ The temperature at the end of the plate width becomes T ３ In other words, the temperature rise at the center of the strip width is ΔT Ｃ is T ２ -T １ In addition, the temperature rise at the width end of the plate is ΔT Ｅ is T ３ -T １ At this time, ΔT Ｅ and ΔT Ｃ have the relationship of the following formula (1).

[0060]

[0061] If the induction heating device 9 is not equipped with equipment such as an edge mask to prevent overheating of the widthwise end portion of the sheet, α is approximately 2. The value of α can be empirically determined, for example, from laboratory experiments or accumulated manufacturing conditions and results. If the induction heating device 9 raises the temperature of the widthwise center portion of the sheet by 100°C (ΔT Ｃ When the temperature of the strip width end portion is increased by 100°C (ΔT Ｅ Therefore, just before the steel sheet is conveyed to the induction heating device 9, the widthwise end portion of the sheet is cooled by the cooling device 13 by an amount sufficient to eliminate the temperature difference (in this example, about 100°C). ＥＡ have the relationship of the following formula (2).

[0062]

[0063] Here, T Ｓ is the temperature of the cooling gas. ＥＡ is the temperature at the width end of the sheet after passing through the cooling device 13. T ＥＢ is the temperature of the width edge of the plate before passing through the cooling device 13. h is the heat transfer coefficient of gas cooling (W / (m ２ S is the surface area (m ２ ) where c is the specific heat of the steel plate (J / (kg·K)). ρ is the density of the steel plate (kg / m ３ ) V is the volume (m ３ ) where t is the time (s) that the steel plate takes to pass through the cooling device 13.

[0064] When cooling the widthwise end of the plate by 100°C, the temperature of the widthwise end of the plate before cooling, T ＥＢ is assumed to be 800°C, and T ＥＡ The cooling conditions for achieving 700°C are calculated.

[0065] Here, the cooling amount can be changed by controlling the temperature or flow rate of the gas used for cooling. While there are no particular limitations on the method for changing the cooling amount, it is particularly desirable to control the flow rate by changing the gas ejection pressure. As a specific example, the relationship between the heat transfer coefficient of the cooling device and a controllable parameter (e.g., gas flow rate or pressure) may be acquired in advance and compiled into a table, and the control parameters of the cooling device 13 may be changed so that a target cooling amount is obtained at the width end of the plate. For example, if the variation in the γ fraction is within a range of 10% at all positions in the width direction of the steel plate, the uniformity of the material is ensured.

[0066] For example, Japanese Patent Application Laid-Open No. 7-41868 (1995) discloses a method for preventing overheating of the steel sheet edge by arranging cooling pipes through which air or water flows as a cooling medium on the inner surface of the furnace wall in a non-oxidizing furnace using a flame burner. With this type of indirect cooling, even if cooling conditions such as the flow rate of the refrigerant are changed, it takes time for the temperature of the fluid in the furnace to change. Therefore, when the heat capacity of the sheet changes, such as when the sheet passing speed or thickness is changed, it takes some time for the target sheet temperature to be reached. In contrast, with the direct cooling using gas disclosed herein, changes in cooling conditions directly lead to changes in the steel sheet temperature. Therefore, even if an unsteady change in heat capacity occurs, it is possible to respond immediately, stably control the sheet temperature at the steel sheet edge, and produce high-quality steel sheet with no material variation across the width.

[0067] (Target Material) The continuous annealing equipment according to this embodiment anneals steel materials including DP steel to stably obtain the target mechanical properties. Specific examples of the target steel sheet include, in weight percent, C of 0.05% or more and 0.25% or less, Si of 0.01% or more and 2.00% or less, Mn of 0.50% or more and 4.00% or less, P of 0.100% or less, S of 0.0500% or less, and Sol. Examples of such steel sheets include those containing 0.005% or more and 0.100% or less of Al, 0.100% or less of N, and optionally containing Cr, Cu, Ni, Sb, Sn (all 1.00% or less), Mo, V, Ti, Nb (all 0.50% or less), Ta (0.10% or less), Mg, Zr (all 0.050% or less), B, Ca, and REM (all 0.0050% or less), with the balance being Fe and unavoidable impurities. However, the steel sheet to be used is not limited as long as it is necessary to control the two-phase structure of α phase and γ phase.

[0068] (Examples) Table 1 shows the conditions and results of producing thin steel sheets using a conventional continuous annealing facility and the continuous annealing facility according to this embodiment (see FIGS. 1 and 2). For Comparative Example 1, Comparative Example 2, Example 1, and Example 2, three types of strength levels, 780 MPa, 980 MPa, and 1180 MPa, were produced, and three types of thicknesses, 1.0, 1.5, and 2.0 mm, were produced for each strength level, with ten slabs produced for each strength level. The ten slabs of each thickness for each grade were all cast in different lots using a continuous casting machine. Therefore, although within the production control range, the chemical compositions of each slab varied, and the transformation behavior was not uniform.

[0069]

[0070] Each slab was hot-rolled and pickled using a conventional method, and annealed and cold-rolled as necessary. Then, it was heat-treated using a conventional continuous annealing facility and the continuous annealing facility disclosed herein. Then, post-treatments such as cooling and plating were performed. Three locations were selected arbitrarily in the steel sheet conveying direction (longitudinal direction) of the final product coil, and tensile test specimens were taken from the center of the sheet width and from both end portions of the sheet width (50 mm from both edges toward the center), and mechanical properties were measured. In other words, a total of nine test specimens were taken from each final product coil, and mechanical properties were measured. The tensile test specimens used were JIS No. 5. The tensile test was performed in accordance with JIS Z2241. The material quality was determined to be stable when the tensile strength (TS) measured by the tensile test varied within a range of 60 MPa, and varied when the TS varied more than a range of -30 MPa to +30 MPa. Furthermore, in the 780 MPa, 980 MPa, and 1180 MPa grades, the required strength ranges are 780 MPa or more, 980 MPa or more, and 1180 MPa or more, respectively.

[0071] Comparative Example 1 was manufactured from the above material using a conventional annealing furnace consisting of a preheating zone 5, a heating zone 6, and a soaking zone 7, without an induction heating device 9 or a transformation rate meter 10. The line speed during manufacturing was in the range of 60 to 120 mpm, and the change in steel sheet temperature was controlled according to the sheet thickness. The steel sheet temperature at the exit of the heating zone 6 (direct flame heating) was set to a range of 600 to 700°C to control the surface oxidation-reduction reaction. Further heating was then performed in the radiant tube soaking zone 7. The steel sheet temperature at the exit of the soaking zone 7 (entrance to the cooling zone 8) was in the range of 770 to 870°C. The furnace temperature of the soaking zone 7 was controlled so that the steel sheet temperature at the exit was the target value. Subsequently, multiple material test pieces were taken from the final product after cooling, hot-dip galvanizing, alloying, etc., and the variation in mechanical properties was investigated. As a result, the tensile strength varied greatly among grades, with some specimens falling below the lower limit for each grade. This is thought to be due to the phase fraction during annealing not reaching the target value due to variations in the chemical composition of the material, and also because when the annealing conditions changed at the joints of coils with different thicknesses and grades, there were areas where the heating conditions could not keep up.

[0072] Comparative Example 2 was produced using the continuous annealing equipment shown in FIG. 1 , but without using the cooling device 13. Therefore, the cooling device is indicated as "none" in Table 1. The transformation rate meter 10 was located in the middle of the cooling zone 8, i.e., between the first cooling zone 8A and the second cooling zone 8B. The strip threading speed and the outlet temperature of the heating zone 6 were controlled within the same ranges as in Comparative Example 1. However, the phase fraction at the width center during annealing was measured using the transformation rate meter 10 at the outlet of the cooling zone 8, and the output of the induction heating device 9 and the furnace temperature of the soaking zone 7 were controlled accordingly. Active control of the phase fraction significantly reduced the variation in mechanical properties at the width center of the product. However, because the cooling device 13 was not used, the width edge portions were overheated by the induction heating device 9, resulting in greater variation in material properties at the width edge portions than in Comparative Example 1.

[0073] Example 1 shows the results of manufacturing a product using the continuous annealing equipment shown in FIG. 1 and the cooling device 13. The transformation rate meter 10 was located in the middle of the cooling zone 8, i.e., between the first cooling zone 8A and the second cooling zone 8B. The phase fractions at the width center and width ends were measured using the transformation rate meter 10, and the output of the induction heating device 9, the furnace temperature of the soaking zone 7, and the cooling amount of the cooling device 13 on the inlet side of the induction heating device 9 were appropriately controlled in accordance with the control flow shown in FIG. 4. Based on the measured phase fractions at the width ends, an appropriate cooling amount of the cooling device 13 could be set, and the material quality variation at both the width center and width ends was significantly improved. Furthermore, the material quality variation at the width ends became comparable to that at the width center.

[0074] Example 2 shows the results of manufacturing a product using the continuous annealing equipment shown in FIG. 2 and a cooling device 13. Here, the transformation rate meter 10 was installed immediately after the induction heating device 9. The transformation rate meter 10 measured the phase fractions at the width center and width end portions, and controlled the output of the induction heating device 9, the furnace temperature of the soaking zone 7, and the cooling amount of the cooling device 13 on the inlet side of the induction heating device 9. Based on the measured phase fractions at the width end portions, an appropriate cooling amount of the cooling device 13 could be set, and the material quality variation at both the width center and width end portions was significantly improved. The material quality variation at the width end portions became comparable to that at the width center. Furthermore, installing the transformation rate meter 10 immediately after the induction heating device 9 made it possible to measure the γ fraction closer to the end of annealing, allowing for quick adjustment of operating conditions. Therefore, the material quality variation was reduced compared to Example 1.

[0075] As described above, the continuous annealing equipment, the continuous annealing method, the method for manufacturing a cold-rolled steel sheet, and the method for manufacturing a plated steel sheet according to the present embodiment can suppress the occurrence of material variations in the sheet width direction by virtue of the above-described configurations. Furthermore, the continuous annealing equipment according to the present embodiment uses an induction heating device 9 that has high heating capacity and fast response to temperature control, thereby making it possible to reduce the line length compared to conventional equipment, and therefore reducing the introduction cost of the manufacturing equipment.

[0076] Although the embodiments of the present disclosure have been described based on the drawings and examples, it should be noted that those skilled in the art would easily be able to make various modifications or alterations based on the present disclosure. Therefore, it should be noted that these modifications and alterations are included within the scope of the present disclosure. For example, the functions included in each component or step can be rearranged so as not to cause logical inconsistencies, and multiple components or steps can be combined or divided into one. The embodiments of the present disclosure can also be realized as a program executed by a processor included in an apparatus or a storage medium on which a program is recorded. It should be understood that these are also included within the scope of the present disclosure.

[0077] Although the zinc pot 11 has been described as a galvanizing bath in which the steel sheet is immersed in the above embodiment, other coating processes may be performed, such as electrogalvanizing, hot-dip galvanizing, or galvannealing.

[0078] For example, a program stored in a storage unit (e.g., a memory) of the process computer may be read and executed by the processor of the process computer to perform processes such as controlling the output of the induction heating device 9, the furnace temperature of the soaking zone 7, and the cooling amount of the cooling device 13. In other words, the processor of the process computer may function as the above-mentioned control device by the program.

[0079] REFERENCE SIGNS LIST 1 Payoff reel 2 Welding machine 3 Electrolytic cleaning device 4 Inlet looper 5 Preheating zone 6 Heating zone 7 Soaking zone 8 Cooling zone 8A First cooling zone 8B Second cooling zone 9 Induction heating device 10 Transformation rate meter 11 Zinc pot 13 Cooling device

Claims

1. A continuous annealing facility for steel sheets comprising a heating zone, a soaking zone, and a cooling zone in this order, an induction heating device provided between the soaking zone and the cooling zone; a cooling device that cools the end of the steel plate by gas at an inlet of the induction heating device; a control device that controls operating conditions of the induction heating device and the cooling device based on a phase fraction of the steel sheet downstream of the induction heating device.

2. The continuous annealing facility according to claim 1 , wherein the phase fraction is measured using a transformation rate meter installed downstream of the induction heating device.

3. A continuous annealing method carried out in a continuous annealing facility for steel sheets comprising a heating zone, a soaking zone, and a cooling zone in this order, performing rapid heating by an induction heating device provided between the soaking zone and the cooling zone; cooling an end of the steel plate at an inlet of the induction heating device by a cooling device; and controlling operating conditions of the induction heating device and the cooling device based on a phase fraction of the steel sheet downstream of the induction heating device.

4. The continuous annealing method according to claim 3, wherein the induction heating device heats the steel sheet at 10° C. / s or more and 200° C. / s or less.

5. The method for continuous annealing a steel sheet according to claim 3, wherein the phase fraction is measured using a transformation rate meter installed downstream of the induction heating device.

6. The method for continuous annealing a steel sheet according to claim 4, wherein the phase fraction is measured using a transformation rate meter installed downstream of the induction heating device.

7. A method for producing a cold-rolled steel sheet, comprising annealing the steel sheet, which is a cold-rolled steel sheet, by the continuous annealing method according to any one of claims 3 to 6.

8. A method for producing a plated steel sheet, comprising the step of performing a plating treatment on a surface of the steel sheet annealed by the method for producing a cold-rolled steel sheet according to claim 7.

9. The method for producing a plated steel sheet according to claim 8, wherein the plating treatment is an electrolytic galvanizing treatment, a hot-dip galvanizing treatment, or a hot-dip galvannealing treatment.