Continuous annealing facility, continuous annealing method, method for producing cold-rolled steel sheet, and method for producing plated steel sheet

Abstract

Provided are 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, which are capable of quickly responding to fluctuations in blank characteristics and minimizing fluctuations in mechanical characteristics of a product. The continuous annealing facility for a steel plate is configured by sequentially comprising a heating zone (6), a soaking zone (7) and a cooling zone (8). The continuous annealing facility is provided with at least one induction heating device (9) arranged between the soaking zone (7) and the cooling zone (8), and at least one measuring device (phase change meter (10)) for measuring the phase fraction of the steel plate after the outlet of the induction heating device (9).

Description

Technical Field

[0001] The present disclosure relates to a continuous annealing device, 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 a continuous annealing device, a continuous annealing method, a method for manufacturing a cold-rolled steel sheet, and a method for manufacturing a plated steel sheet for manufacturing high-strength steel sheets used in automotive structural materials and the like. Background Art

[0002] In the production of thin steel sheets for automobiles, the continuously cast slabs are extensively processed by hot rolling and cold rolling before reaching the final thickness. The subsequent annealing treatment recovers the cold worked structure, recrystallizes and grains grow, and controls the phase transformation structure to adjust the balance between strength and workability.

[0003] In recent years, in the annealing process, a continuous annealing furnace is generally used, which continuously heats, soaks and cools the steel sheets while conveying the steel sheets connected in a strip shape. In addition, hot-dip galvanizing and over-aging treatment are performed after the cooling, depending on the use of the steel sheets.

[0004] The heating mechanism of the annealing furnace is generally a radiant tube burner that heats a metal tube (radiant tube) by gas combustion and indirectly heats the steel sheet by its radiant heat. In addition, in the manufacture of hot-dip galvanized steel sheets, in order to ensure the coating properties, a direct-fire furnace is sometimes used in the front section of the heating furnace to heat the steel sheet by directly spraying the flame of gas combustion onto the steel sheet.

[0005] Since the radiant tube furnace uses the radiant heat from the radiant tube and the furnace wall to heat the steel plate, the volume of the heat source is very large and the thermal inertia is large. Therefore, it is difficult to quickly follow the changes in the set temperature. In addition, in the final stage of heating, the temperature rise rate of the steel plate is slow, and a certain soaking time is required to control the structure, so the required furnace length is extended, the thermal inertia becomes larger, and the tracking of the target temperature is delayed. As a result, during the continuous processing of the coil, the temperature of a part of the steel plate is not within the specified annealing temperature range, which will cause problems such as reduced yield due to deviations in mechanical properties and reduced productivity due to changes in the production line speed used to control the temperature.

[0006] In response to the above-mentioned problems, Patent Document 1 discloses a technology for improving responsiveness by providing an induction heating device between a preheating zone and a heating zone (direct-fire heating furnace) or in the middle of a heating zone composed of multiple direct-fire heating furnaces to compensate for the heating capacity. Patent Document 2 discloses a technology for performing continuous annealing at a high efficiency by providing an electric heating device or an induction heating device at the outlet of the soaking zone. Patent Document 3 discloses the following technology for slowly heating at 650°C to 750°C and then rapidly heating to a temperature below the maximum annealing temperature of 750°C to 910°C. Patent Document 4 discloses a technology for stabilizing the deviation of mechanical properties caused by fluctuations in steel composition and hot rolling conditions by varying the maximum annealing temperature in the long side direction of the steel plate using a recrystallization / grain growth model.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 11-061277

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 09-003552

[0011] Patent Document 3: Japanese Patent Application Publication No. 2000-144262

[0012] Patent Document 4: Japanese Patent Application Laid-Open No. 10-152728 Summary of the invention

[0013] However, in order to prevent excessive oxidation of the steel plate surface, the technology of Patent Document 1 has a low temperature of the steel plate at the outlet of the direct-fired heating furnace, and it is necessary to heat it with a radiation tube furnace. Therefore, it is difficult to shorten the furnace length, and the influence range of the furnace temperature setting change is wide. In addition, although the influence of furnace temperature fluctuation can be absorbed by high-speed control of the output of the induction heating device, the direct-fired heating furnace after the induction heating device focuses on controlling the oxidation of the steel plate, and it is difficult to flexibly control the temperature of the steel plate. Therefore, the effect of absorbing the influence of furnace temperature fluctuation cannot be fully exerted.

[0014] The technology of Patent Document 2 targets bake hardened (BH) steel sheets, and claims that by installing a heating device at the outlet of the soaking zone, a high annealing temperature can be achieved without reducing the conveying speed even with a small annealing furnace. In addition, the technology of Patent Document 2 claims that the best material can be obtained by introducing a function related to the steel sheet temperature called the annealing parameter and making the annealing parameter higher than a threshold. However, in recent years, due to the demand for higher strength of products, dual-phase steel (DP steel) with an increased production volume needs to be transformed from the α phase to the γ phase during annealing. In the case of DP steel, it is necessary to heat to A in the annealing furnace. 1 Above the phase transition point and less than A 3The highest temperature at which the transformation point is reached is used, and the fractions of the α phase and the γ phase are controlled according to the reached temperature. Although the control of the annealing temperature is an important parameter in the manufacture of DP steel, the fractions of the α phase and the γ phase are the primary indicators for controlling the material quality. Therefore, the technology of Patent Document 2 may not necessarily stabilize the material quality of the steel plate.

[0015] The technology of Patent Document 3 is for interstitial-free steel (IF steel), and controls the maximum annealing temperature by rapidly heating with an induction heating device or an electric heating device before soaking, and absorbs the material deviation predicted from the actual result of the previous process by the change in the grain size of the α recrystallized grains after annealing. However, in the case of DP steel, if the temperature is rapidly increased to the soaking temperature, i.e., the maximum reaching temperature, after slow heating as in the technology of Patent Document 3, the α phase rapidly transforms into the γ phase while the grain size remains fine, and thus the target phase fraction cannot be controlled.

[0016] The technology of Patent Document 4 heats the steel sheet to the recrystallization region and then temporarily holds it in the region to allow it to fully recrystallize and then rapidly heat it again to the target annealing temperature. If this method is used, it is possible to prevent the rapid phase transition from the α phase to the γ phase caused by rapid heating, and in principle it is possible to control the phase fraction by controlling the maximum reached temperature. However, in the actual manufacturing process, the phase transition behavior fluctuates according to the fluctuation of the chemical composition of the material, so it is difficult to control the phase fraction simply by the temperature of the steel sheet, and it is impossible to control the material differences of each product.

[0017] In view of the above problems, the present disclosure aims to provide a continuous annealing device, a continuous annealing method, a method for manufacturing a cold-rolled steel sheet, and a method for manufacturing a plated steel sheet that can quickly respond to fluctuations in billet properties and minimize fluctuations in mechanical properties of products.

[0018] (1) A continuous annealing facility according to one embodiment of the present disclosure is a continuous annealing facility for steel plates that includes a heating zone, a soaking zone, and a cooling zone in sequence, and includes:

[0019] at least one induction heating device disposed between the soaking zone and the cooling zone, and

[0020] At least one measuring device for measuring the phase fraction of the steel plate is provided after the outlet of the induction heating device.

[0021] (2) As one embodiment of the present disclosure, in (1), a control device is provided for determining the operating conditions of the induction heating device based on the measurement result of the phase fraction measuring device.

[0022] (3) As one embodiment of the present disclosure, in (1) or (2), the heating rate of the induction heating device is 10° C. / sec to 200° C. / sec.

[0023] (4) A continuous annealing method according to an embodiment of the present disclosure is a continuous annealing method for a steel plate which is sequentially conveyed in a heating zone, a soaking zone, and a cooling zone, comprising the following steps:

[0024] After the temperature of the steel plate is maintained in the recrystallization temperature range in the soaking zone, the steel plate is rapidly heated by at least one induction heating device disposed between the soaking zone and the cooling zone; and

[0025] A step of controlling the operating conditions of the induction heating device based on the phase fraction of the steel plate obtained between an outlet of the soaking zone and an inlet of the cooling zone.

[0026] (5) As one embodiment of the present disclosure, in (4), the steel plate is heated at 10° C. / sec to 200° C. / sec by the induction heating device.

[0027] (6) A method for producing a cold-rolled steel sheet according to one embodiment of the present disclosure includes the step of annealing the steel sheet as the cold-rolled steel sheet at an annealing temperature adjusted by the continuous annealing method of (4) or (5).

[0028] (7) A method for producing a plated steel sheet according to one embodiment of the present disclosure includes the step of subjecting the surface of the steel sheet annealed by the method for producing a cold-rolled steel sheet according to (6) to a plating treatment.

[0029] (8) As one embodiment of the present disclosure, in (7), the plating treatment is electrogalvanizing, hot-dip galvanizing, or alloy hot-dip galvanizing.

[0030] According to the present disclosure, a continuous annealing device, a continuous annealing method, a method for manufacturing a cold-rolled steel sheet, and a method for manufacturing a plated steel sheet are provided, which can quickly respond to fluctuations in billet properties and minimize fluctuations in mechanical properties of products. Therefore, compared with conventional annealing furnaces, thin steel sheets having target mechanical properties can be stably manufactured. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram showing a hot-dip galvanizing process provided with a continuous annealing facility according to one embodiment.

[0032] Figure 2 This is a diagram showing an example of the temperature history of a steel plate.

[0033] Figure 3 This is a flow chart showing an example of a continuous annealing method. DETAILED DESCRIPTION

[0034] Hereinafter, a continuous annealing facility, a continuous annealing method, a method for producing a cold-rolled steel sheet, and a method for producing a plated steel sheet according to an embodiment of the present disclosure will be described with reference to the drawings.

[0035] <Facility Configuration>

[0036] Figure 1 A part of the hot-dip galvanizing process of the continuous annealing equipment of this embodiment is shown. In this embodiment, the steel material manufactured by the hot-dip galvanizing process is a thin steel plate. In addition, the manufactured steel material is a cold-rolled steel plate. Figure 1 The arrow indicates the direction of travel of the production line. In the following, the upstream side of the travel direction is sometimes expressed as "front" and the downstream side is expressed as "rear". The continuous annealing equipment includes a uncoiler 1, a welding machine 2, an electrolytic cleaning device 3, an inlet-side looper 4, a preheating zone 5, a heating zone 6, a soaking zone 7 and a cooling zone 8. In addition, the continuous annealing equipment includes an induction heating device 9 (Induction Heating device, sometimes referred to as "IH" below) and a phase transformer 10. The continuous annealing equipment includes at least one induction heating device 9. In addition, the continuous annealing equipment includes at least one phase transformer 10. In the present embodiment, the cooling zone 8 includes a first cooling zone 8A and a second cooling zone 8B. In addition, the continuous annealing equipment may further include a galvanizing tank (zinc pot 11) for immersion cooling of thin steel plates to a specified temperature, an alloying zone, a holding zone, a final cooling zone, a tempering rolling device, an outlet-side looper, a tension coiler, and the like.

[0037] The thin steel plate that has been coiled in the previous process is unwound by the uncoiler 1. The unwound thin steel plate passes through the preheating zone 5 and enters the continuous annealing furnace. The continuous annealing equipment is composed of 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. In addition, the continuous annealing equipment is provided with a phase change meter 10 after the outlet of the induction heating device 9. Figure 1 In the example, the phase change meter 10 is disposed between the induction heating device 9 and the cooling zone 8, but the position is not limited as long as it is disposed after the outlet of the induction heating device 9. Here, the phase change meter 10 is an example of a measuring device for measuring the phase fraction of the steel plate.

[0038] The continuous annealing equipment is configured to include a control device for adjusting the output of the induction heating device 9 using the measurement result of the phase fraction of the phase change analyzer 10. Figure 1The structure can be modified according to the target quality of the steel plate to be manufactured. For example, a zinc pot 11 and an alloying belt can be set after the continuous annealing furnace, or the zinc pot 11 can be omitted. In addition, the continuous annealing equipment can be divided into a part of the equipment. For example, the soaking zone 7 can be divided so that the divided soaking zone 7 can control the atmosphere or temperature independently.

[0039] <Details of components>

[0040] Figure 2 The temperature history of a steel plate when annealing is performed using an annealing furnace of the prior art and when annealing is performed using an annealing furnace of the present embodiment is illustrated. The vertical axis is the temperature of the steel plate. The horizontal axis is the passing time. The production line speed is 100 mpm. In addition, the thickness of the steel plate is 1 mm. The temperature history in the present embodiment is represented by a solid line, and the corresponding process (equipment) is recorded below. In addition, the temperature history of the prior art is represented by a dotted line, and the corresponding process (equipment) is recorded above. Figure 2 As shown, in the annealing furnace of the present embodiment, the time required for annealing is also shortened compared with the conventional technology by the configuration described below.

[0041] (Pre-heat zone)

[0042] The steel sheet fed from the uncoiler 1 at room temperature to about 100° C. first enters the preheating zone 5 and is heated to about 200° C. The temperature of the steel sheet can be simply increased by heating. In this embodiment, the preheating zone 5 uses the high-temperature exhaust gas generated in the heating zone 6 .

[0043] (Heating belt)

[0044] Next, the steel plate enters the heating zone 6, and the temperature of the steel plate is heated to about 600-700°C. The temperature of the thin steel plate can be simply increased by heating. In this embodiment, the heating zone 6 adopts a direct-fire heating furnace method in order to heat up to a certain temperature in a short time and control the surface state. The direct-fire heating furnace not only has a high heating capacity and can reduce the volume of the furnace, but also can flexibly control the redox reaction on the surface of the steel plate in consideration of the subsequent plating process.

[0045] (tropical zone)

[0046] The main function of soaking zone 7 is to recrystallize the α phase. However, at this time, some 1Phase transition point. As a heating method for the soaking zone 7, from the perspective of high efficiency and heating uniformity, it is preferred to adopt a radiation heating method based on gas combustion (radiant tube heating). The soaking zone 7 can be composed of a larger single furnace shell. In addition, the soaking zone 7 can be composed as follows: it has more than two partitions, each partition is separated by an insulating wall, etc., and is maintained and heated within a target temperature range determined by each partition. In this embodiment, the soaking zone 7 is set at a temperature of the steel plate lower than A 1 The temperature range of the phase transformation point (about 600 to 730°C) is maintained or slowly heated to ensure the residence time in the recrystallization temperature range and to suppress the residual non-recrystallized α phase. 1 The phase transformation point is the temperature at which the austenite phase transformation occurs. As an example, the furnace temperature is set to 730°C. In other words, at A 1 Austenite phase begins to appear at temperatures above the phase transformation point.

[0047] The recrystallization temperature range is the temperature range where metal microstructure recrystallization occurs. In this application, it refers to 600°C to A 1 The temperature range of the phase transition point. Especially A 1 The specific temperature of the phase transition point can be obtained by an empirical formula using the generally known alloy component content, or by experiments using materials with average alloy components of each product in advance, or by using a regression equation based on the alloy component content based on these experimental results. The regression equation based on the alloy component can use a generally known empirical formula.

[0048] Here, if the temperature of the steel sheet in the soaking zone 7 is too low, the recrystallization of the α phase will not proceed, and sufficient workability cannot be obtained. 1 Above the transformation point, the α recrystallized grains become coarse and the strength is insufficient. 1 The transformation point may slightly fluctuate depending on the composition of the steel sheet. Therefore, the recrystallization temperature range is preferably estimated in advance by measurement, calculation or simulation, and is set in consideration of the error range in annealing.

[0049] If the retention time of the steel plate in the soaking zone 7 is too short, the recrystallization of the α phase will not proceed sufficiently. If it is too long, the grains will become coarse, resulting in reduced mechanical properties. Therefore, the optimal retention time was studied in advance, and it was found that it needs to be around 15 to 80 seconds. This is because if it is less than 15 seconds, the recrystallization of the α phase will not proceed sufficiently, and the workability will deteriorate. If it exceeds 80 seconds, local coarse grains will be generated, and the strength will become uneven. Therefore, in the soaking zone 7, the temperature of the steel plate is below A 1 The temperature region of the phase transition point is maintained for 15 seconds to 80 seconds.

[0050] When slow heating is performed in the soaking zone 7, the residence time in the recrystallization temperature region only needs to ensure the above time. However, when slow heating is performed in the soaking zone 7, the heating rate is preferably 5°C / second or less. This is because if rapid heating is performed exceeding 5°C / second, it is impossible to suppress the retention of the non-recrystallized α phase.

[0051] In the soaking zone 7, after ensuring the retention time in the recrystallization temperature range, if the range is less than 750°C, even in the A 1 Heating may be performed more slowly at a temperature above the phase transition point, but the heating rate is preferably 5° C. / sec or less.

[0052] Although the continuous annealing equipment of the present embodiment is provided with a soaking zone 7 of a radiant tube furnace, it may be provided with a structure without such a soaking zone 7 as long as it has a function of a heat retention time sufficient for the recrystallization of the α phase. For example, the soaking zone 7 is not limited to a radiant tube furnace. In addition, the continuous annealing equipment may not be provided with the soaking zone 7, but may have a function of ensuring that the heating zone 6 has a heat retention time sufficient for the recrystallization of the α phase. In addition, the continuous annealing equipment may be provided with a heat retention device instead of the soaking zone 7. In the case where the continuous annealing equipment is not provided with the soaking zone 7, the induction heating device 9 only needs to be provided between the portion having the function of the soaking zone 7 (in the above example, the portion or heat retention device that ensures the heat retention time of the heating zone 6) and the cooling zone 8.

[0053] (Induction Heating Device)

[0054] Next, the induction heating device 9 adjusts the output so that the temperature of the steel plate is within A 1 Above the phase transition point and below A 3 The steel sheet is rapidly heated in the temperature region of the phase transformation point (about 750 to 900°C). 3 The phase transformation point is the upper limit temperature that can suppress the γ phase fraction. The purpose of this process is to uniformly heat the entire steel plate to A in a short time. 1 Temperatures above the phase transition point also contribute to the miniaturization of the entire device. 1 Phase transition point and A 3 The transformation point may slightly fluctuate depending on the composition of the steel sheet. Therefore, the appropriate temperature range is preferably estimated in advance by measurement, calculation or simulation, and is set in consideration of the error range in annealing.

[0055] In DP steel, which is one of the objects of the continuous annealing equipment of this embodiment, the reaching temperature of the induction heating device 9 has a great influence on the mechanical properties of the final product. Therefore, the heating method of the induction heating device 9 needs to respond quickly to the temperature control command. In addition, when heating in such a temperature range, since the Curie point of the magnetic change of the steel plate is exceeded, it is preferred that the induction heating device 9 is a horizontal type.

[0056] In addition, since the recrystallization region of the α phase is maintained in the soaking zone 7, if the heating takes time, it will lead to the coarsening of the α grains, so an induction heating device 9 that can heat quickly is also preferred. The induction heating device 9 can heat up at 10°C / second to 200°C / second. This is because when it is less than 10°C / second, it will lead to the coarsening of the α grains. If it is greater than 200°C / second, a local high-temperature portion will be generated in the width direction and uniformity cannot be maintained. The induction heating device 9 is more preferably heated at 20°C / second to 100°C / second. This is because if the heating rate is more than 20°C / second, the length of the production line can be further shortened, and if it is less than 100°C / second, the risk of buckling and deformation of the steel plate due to thermal stress can be further reduced.

[0057] If the induction heating device 9 is used to heat quickly to the target annealing temperature, the phase transformation from the α phase to the γ phase immediately after heating will not reach the equilibrium state. However, if it is kept near the target annealing temperature for too long, the phase transformation from the α phase to the γ phase will proceed to a higher degree than necessary, making the material control complicated. Therefore, after reaching the target annealing temperature, it is best to enter the cooling zone 8 as quickly as possible with a holding time of 0 seconds, and it is preferred to start cooling within 5 seconds. In order to avoid a non-equilibrium state, even if it is kept, cooling must be started within 10 seconds at the latest. That is, the steel plate starts cooling in the cooling zone 8 within 10 seconds after the heating of the induction heating device 9 is completed.

[0058] In order to satisfy such conditions, the induction heating device 9 is preferably provided at the connection portion between the soaking zone 7 and the cooling zone 8. Providing the induction heating device 9 at the connection portion may also be done by adding the induction heating device 9 to the existing furnace body.

[0059] (Phase change instrument)

[0060] A phase change meter 10 for measuring the phase fraction of the steel plate is arranged on the outlet side (after the outlet) of the induction heating device 9, and the phase fractions (phase transformation rate) of the α phase and the γ phase of the steel plate after reaching the target annealing temperature are measured. In the phase change meter 10, it is particularly effective in the present invention to grasp the γ phase fraction. Furthermore, based on the γ phase fraction measured by the phase change meter 10, it is set to further appropriately fine-tune the pre-set operating conditions of the induction heating device 9 and the cooling belt 8. Through such adjustments, the mechanical properties of the steel plate can be stably obtained. Here, as for the setting position of the phase change meter 10, if it is after the outlet of the induction heating device 9, it is not particularly limited. However, from the perspective of feedback of the measured value for controlling the induction heating device 9 and the cooling belt 8, it is preferably as close to the induction heating device 9 as possible. Specifically, it is preferably set between the outlet of the induction heating device 9 and the inlet of the cooling belt 8.

[0061] Here, the method for measuring the phase transformation rate is not particularly limited. As a magnetic detector, that is, a device for measuring the magnetic phase transformation rate of the steel strip, a magnetic phase transformation rate measuring device composed of a driving coil for generating a magnetic field and a detection coil for measuring the magnetic field passing through the steel strip can be used to measure the austenite fraction. Specifically, the device described in Japanese Patent Gazette No. 2019-7907 can be used. However, in the case of a magnetic phase transformation rate measuring device, if the phase change meter 10 is set near the induction heating device 9, it will be affected by the magnetic field and the measurement accuracy will be reduced. Therefore, the magnetic phase transformation rate measuring device can be set inside the soaking zone 7 or at the outlet of the cooling zone 8, etc., instead of immediately after the induction heating device 9. As a method that is not affected by the magnetic field, a device that applies the X-ray diffraction method can be used. Due to the difference in crystal structure, the γ phase and the α phase generate diffraction peaks at inherent angles when the steel plate is irradiated with X-rays. The γ phase fraction can be quantified according to the intensity of the diffraction peak. Among the commercially available devices, for example, X-CAP manufactured by SMS can be cited.

[0062] (Cooling Zone)

[0063] The cooling zone 8 is a device for cooling the steel plate to a predetermined temperature, and as a cooling mechanism, jet cooling, roller cooling, water cooling (water quenching), etc. can be used. As in the present embodiment, the cooling zone 8 can be divided into a plurality of zones such as the first cooling zone 8A and the second cooling zone 8B, and different cooling mechanisms can be combined, and the cooling conditions of the same type of cooling mechanisms can be changed to control the thermal history of the steel plate during cooling.

[0064] (Hot dip galvanizing bath)

[0065] A hot dip galvanizing bath may be provided after the cooling zone 8, and hot dip galvanizing may be performed on the steel sheet discharged from the cooling zone 8. Hot dip galvanizing may be performed according to a conventional method, and a nozzle, a roll in the bath, etc. may be provided as necessary.

[0066] (Alloying equipment)

[0067] An alloying treatment device may be provided after the hot-dip galvanizing bath. The alloying treatment device is used to heat the steel sheet to perform the alloying treatment. The alloying treatment may be performed according to a conventional method.

[0068] (Other equipment)

[0069] In addition to the alloying equipment, in order to improve the quality of the final product and achieve manufacturing stability and efficiency, it is possible to further have a heat preservation zone, a final cooling zone, a tempering rolling equipment, a straightening machine, an exit side looper, a tension coiler, etc. These equipments can be installed and used according to the quality required for the product, and there is no particular limitation.

[0070] <Operation method (control method)>

[0071] In this embodiment, the range of the optimum phase fraction is known in advance for each product, and the operating conditions of the annealing furnace are controlled in such a way that the measurement result of the phase fraction based on the phase change meter 10 falls within the target range. As the control object, in addition to the furnace temperature of the soaking zone 7 and the output of the induction heating device 9, the operating conditions of the cooling zone 8 or the alloying zone set on the downstream side of the induction heating device 9 can also be used. However, from the viewpoint of responsiveness, the output of the induction heating device 9 is the most effective for material stabilization. In addition, the control method can be manual control by the operator of the device. Specifically, a detector that continuously displays the measurement results of the phase change meter 10 and the target range of the coil can be set in the control room, and the operating conditions of the above-mentioned control object can be manually changed so that the measurement results fall within the target range. However, since there may be a delay or erroneous operation in the condition change during manual control, it is preferred to know the range of the optimum phase fraction for each product in advance and construct a phase change rate control model, and construct a control system that performs automatic feedback control of the induction heating device 9 so that the measurement result of the phase fraction using the phase change meter 10 falls within the target range. In this way, the annealing temperature of the steel sheet set in advance can be appropriately adjusted to manufacture products with very stable mechanical properties. A phase transformation rate control model is constructed for each product. In this embodiment, strength grade is used as a factor for distinguishing products. The factor for distinguishing products is not limited to strength grade. For example, the phase transformation rate control model for each product can be constructed by distinguishing products based on necessary product characteristics such as steel type, mechanical properties other than strength, and surface properties.

[0072] Figure 3 This is a flowchart showing an example of a continuous annealing method performed using the above-mentioned continuous annealing equipment in this embodiment.

[0073] According to the phase fraction measurement results obtained by using the phase change meter 10 in the annealing furnace ( Figure 3 information on the chemical composition obtained when casting the material ( Figure 3The material prediction model is used to predict the mechanical characteristics (mechanical properties) of the final product based on the "material information" of the continuous annealing equipment (especially the operating conditions of the cooling zone 8) and the current operating conditions of the continuous annealing equipment (especially the operating conditions of the cooling zone 8). At this time, the material prediction model used can use not only a physical model obtained through laboratory experiments or numerical analysis in an offline state, but also a machine learning model obtained based on the accumulation of actual manufacturing results.

[0074] Next, determine the predicted mechanical properties ( Figure 3 The target range of mechanical properties is given by the host computer. The host computer is a computer that provides manufacturing information to the process computer that controls the operation of the continuous annealing equipment. If the continuous annealing equipment is within the target range ( Figure 3 If the "Mechanical properties target range" is Yes, the continuous annealing equipment will continue to operate under the current conditions. Figure 3 When the operating conditions are changed, the phase fraction adjustment amount required to make the material fall within the target range is obtained by the material prediction model described above, and the steel plate temperature control amount used to achieve the phase fraction adjustment amount is obtained. The phase transformation rate control model can be a physical model obtained by laboratory experiments or numerical analysis in an offline state, or a machine learning model obtained by accumulating actual manufacturing results.

[0075] When the continuous annealing equipment determines that the required temperature control range is within the range controllable by the induction heating device 9 ( Figure 3 If the "IH output control range" of the continuous annealing equipment is Yes, the output of the induction heating device 9 is changed. If the continuous annealing equipment determines that the required temperature control range is not within the range that can be controlled by the induction heating device 9 ( Figure 3 The operating conditions of the equipment other than the induction heating device 9 are also changed accordingly ( Figure 3 The control object is a device arranged on the upstream side of the phase change instrument 10, such as the cooling zone 8 or the soaking zone 7. The control item may be the injection pressure of the cooling gas in the case of jet cooling in the cooling zone 8, or the furnace temperature in the case of the soaking zone 7. These devices are less responsive to changes in operating conditions than the induction heating device 9, but by controlling them in combination with the induction heating device 9 through the above-mentioned control process, the deviation of the mechanical properties of the product from the target can be suppressed to a minimum.

[0076] Here, in the conventional continuous annealing equipment, a phase change meter 10 may be provided to try to control the phase change rate by high-precision control of the plate temperature. However, in the conventional equipment specifications of only the radiation heating annealing furnace, the heating takes time (e.g., about 100 to 200 seconds), so the responsiveness of the furnace temperature to the control command is very poor. In addition, the length of the steel plate affected by the change in furnace temperature also becomes longer, so it is impossible to accurately control the temperature of the steel plate. On the other hand, in the continuous annealing equipment of the present embodiment, the temperature of the steel plate can be controlled at a high speed (e.g., less than 10, about 5 seconds). In addition, since the phase change to the γ phase does not need to be completed in the soaking zone 7, the furnace length can be shortened (e.g., 70 to 110 m), so the responsiveness to the command of the furnace temperature change accompanying the change in the steel plate size or the change in the steel grade becomes higher. As a result, not only the response to the fluctuation of the phase change behavior caused by the material is fast, but also the change in the heating condition can be quickly reflected when the annealing condition at the product joint changes. Moreover, the range of deviation from the target annealing condition can be narrowed, and the yield rate is greatly improved.

[0077] The steel sheet may be subjected to plating, alloying, temper rolling, and shape correction treatment after annealing in a continuous annealing facility. The plating and alloying treatments may be conventional methods for satisfying the quality required for the surface characteristics of the product, and are not particularly limited.

[0078] In addition, if the shape is irregular, it can be straightened by continuing the temper rolling and passing the plate through the straightening machine. The temper rolling and straightening can be performed without limitation under the condition that only the shape is corrected without affecting the mechanical properties of the steel plate.

[0079] (Target materials)

[0080] The continuous annealing equipment of the present embodiment is an equipment for annealing steel materials including DP steel to stably obtain target mechanical properties. Specific examples of the target steel plate include the following steel plates: containing, by weight%, 0.03% to 0.25% C, 0.01% to 2.50% Si, 0.50% to 4.00% Mn, 0.100% or less P, 0.0500% or less S, 0.005% to 2.000% Sol.Al, 0.100% or less N, and, as required, 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, REM (all 0.0050% or less), and the remainder is Fe and unavoidable impurities. However, the target steel sheet is not limited as long as it is a dual-phase structure in which the α phase and the γ phase need to be controlled.

[0081] (Example)

[0082] Table 1 shows the results of using the conventional continuous annealing equipment and the continuous annealing equipment of the present embodiment (see Figure 1 ) to manufacture thin steel plates and the results. Here, in addition to Comparative Example 4, in order to investigate the deviation of the mechanical properties of the products, 30 sheets of each of the products of multiple strength grades were manufactured in the form of coils. The manufacturing strength grades are 780MPa, 980MPa, and 1180MPa, and the plate thickness is 1.0, 1.5, and 2.0mm. 10 sheets of each strength grade are manufactured. The 10 slabs of each grade and each thickness are all cast in different batches in a continuous casting machine. Therefore, although it is within the scope of manufacturing management, there are deviations in the chemical composition of each slab, and the phase change behavior is also uneven. In Table 1, the equipment configuration shows the devices that constitute the equipment, etc. In addition, in Table 1, FB represents feedback and FF represents feedforward.

[0083]

[0084] Each slab is hot rolled and pickled by the usual method, and annealed and cold rolled as needed, and then heat treated by laboratory annealing, conventional continuous annealing equipment and the continuous annealing equipment disclosed in the present invention, and then subjected to post-treatment such as cooling and plating. Tensile test pieces are taken every 100m in the conveying direction of the steel plate of the final product coil to measure the mechanical properties. Here, the tensile test piece is JIS No. 5. The tensile test is carried out in accordance with JIS Z2241. When the tensile test result relative to the full length of the coil exceeds the lower limit of the target value, the length between 10m before and after the test piece is taken is the material test failure length. The ratio of the material test failure length to the full length of the coil is calculated as the "material test (abbreviation for material test) failure rate". In addition, at each level of 780MPa, 980MPa, and 1180MPa, the required strength range is above 780MPa, above 980MPa, and above 1180MPa, respectively.

[0085] Comparative Example 1 uses 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 and a phase change meter 10, and is manufactured from the above materials. The production line speed during manufacturing is in the range of 60 to 120 mpm, and the heating mode during steel plate annealing is controlled according to the plate thickness and composition. The steel plate temperature at the outlet of the heating zone 6 (direct fire heating) is in the range of 600 to 700°C, and the redox reaction on the surface is controlled. Then, it is further heated by introducing the soaking zone 7 of the radiation tube method, and the residence time in the recrystallization region (about 600 to 730°C) is 15 to 80 seconds. The heating / soaking time below the recrystallization region is 100 to 200 seconds. The steel plate temperature at the outlet of the soaking zone 7 (the entrance of the cooling zone 8) is in the range of 750 to 850°C. The furnace temperature of the soaking zone 7 is controlled in such a way that the steel plate temperature at the outlet reaches the target value. Then, after cooling, hot-dip galvanizing, alloying, etc., multiple material test pieces were taken from the final product to investigate the deviation of mechanical properties. As a result, there were large differences in tensile strength between grades, and there were test pieces that were below the lower limit of each grade, resulting in many parts where the billet properties did not reach the target value. It is speculated that the reason is that the phase fraction during annealing did not reach the target value due to fluctuations in the chemical composition of the material, and that when the annealing conditions changed at the joints of coils with different plate thicknesses and grades, parts where the heating conditions could not follow were generated.

[0086] Comparative Example 2 uses the same conventional annealing furnace as Comparative Example 1 and sets a phase change meter at the outlet of soaking zone 7 to manufacture the final product coil. The basic manufacturing conditions are set in the same way as in Comparative Example 1, and the phase change meter 10 at the outlet of soaking zone 7 is used to measure the phase change rate and control the furnace temperature of soaking zone 7. Since the furnace temperature of soaking zone 7 is changed in accordance with the obtained phase change rate, the fluctuation range of the temperature at the inlet of cooling zone 8 becomes larger than that in Comparative Example 1, but since the furnace temperature of soaking zone 7 is actively controlled, the material test failure rate is improved compared with Comparative Example 1. However, it is estimated that parts that cannot follow the control of the annealing furnace are generated at the joints of coils with different plate thicknesses and the joints of coils with different annealing conditions, and the failure of the material test cannot be suppressed.

[0087] Comparative Example 3 uses the same conventional annealing furnace as Comparative Example 1 and sets a phase change meter at the outlet of the cooling zone 8 to manufacture the final product coil. The basic manufacturing conditions are set in the same way as in Comparative Example 1, and the phase change meter 10 at the outlet of the cooling zone 8 is used to measure the phase change rate, and the furnace temperature of the soaking zone 7 and the refrigerant injection pressure of the cooling zone 8 are controlled. Since the furnace temperature of the soaking zone 7 is changed according to the phase change rate obtained by the reaction, the fluctuation range of the temperature at the inlet of the cooling zone 8 becomes larger than that of Comparative Example 1, but since the furnace temperature of the soaking zone 7 is actively controlled, the material test failure rate is improved compared with Comparative Example 1. In addition, since the refrigerant injection pressure of the cooling zone 8 is also controlled, the material test failure rate is improved compared with Comparative Example 2. However, it is estimated that parts that cannot follow the control of the annealing furnace are generated at the joints of coils with different plate thicknesses and the joints of coils with different annealing conditions, and the material test failure cannot be suppressed.

[0088] Comparative Example 4 is provided with an induction heating device at the exit of the soaking zone 7 of the conventional annealing furnace. The plate passing speed and the temperature of the steel plate at the exit of the heating zone 6 are controlled in the same range as in Comparative Example 1. Although an induction heating device 9 is provided, the measured value of the phase change meter 10 is not fed back to the output control and furnace temperature control of the induction heating device 9, and the steel plate temperature obtained by measurement or calculation in the furnace is controlled as in the past. As a result, the controllability of the steel plate temperature is improved, and the deviation of the mechanical properties of the product is improved, but the fluctuation caused by the chemical composition of the material cannot be absorbed, and the material test failure rate is not greatly improved compared with Comparative Example 1.

[0089] Comparative Example 5 is provided with an induction heating device at the entrance of the soaking zone 7 of the conventional annealing furnace. In addition, a phase transformer 10 is provided at the exit of the soaking zone 7. The plate passing speed and the temperature of the steel plate at the exit of the heating zone 6 are controlled in the same range as in Comparative Example 1. In addition, the operator manually controls the operating conditions of each device of the annealing furnace according to the measurement results of the phase transformer 10. Although the induction heating device 9 is provided, the residence time in the recrystallization zone is very short, and the phase transformation from the α phase to the γ phase is rapidly carried out in a state where the recrystallization is not fully carried out. Therefore, even if the phase transformation rate is measured at the exit of the soaking zone 7, it is still a single γ phase, and there is no change, and the target structure control cannot be achieved at all.

[0090] Comparative Example 6 is an example of applying an automatic feedback control system based on the measurement results of the phase change analyzer 10 to control the operating conditions of the soaking belt 7 and the induction heating device 9 in Comparative Example 5. Although the control method is changed, the phase change from the α phase to the γ phase cannot be controlled as in Comparative Example 5, and the target structure cannot be obtained.

[0091] In Comparative Example 7, a phase change meter 10 is provided at the outlet of the soaking zone 7 of the conventional annealing furnace and an induction heating device 9 is provided at the downstream side thereof. A feedforward control system is adopted to control the operating conditions of the induction heating device 9 at the downstream side based on the phase fraction information measured by the phase change meter 10. However, since the α single phase is almost maintained at the outlet of the soaking zone 7, the output of the induction heating device 9 is almost kept constant, and thus the fluctuation of the mechanical characteristics is not improved.

[0092] Example 1 is to use Figure 1 The results of the continuous annealing equipment of the present disclosure shown in the figure are the results of manufacturing products from the above materials. The plate passing speed and the steel plate temperature at the outlet of the heating zone 6 are controlled in the same range as in Comparative Example 1. However, in Example 1, the phase fraction just after annealing is measured by the phase change meter 10 at the outlet of the induction heating device 9, and the output of the induction heating device 9 at the outlet of the soaking zone 7 and the furnace temperature of the soaking zone 7 are controlled. In this way, by controlling based on the phase fraction, the mechanical properties of the product are stable, and no material test failure occurs over the entire length.

[0093] Example 2 is the same as Example 1. Figure 1 The result of the continuous annealing equipment of the present disclosure shown is the result of manufacturing products from the above materials. However, the setting position of the phase change instrument 10 is changed to the outlet of the cooling belt 8. Since the phase change instrument 10 as a sensor is far away from the induction heating device 9 used as an actuator in priority, the response is delayed compared with Example 1. Since there is a part where the output change of the induction heating device 9 does not follow, although a certain effect is shown in improving the material test failure rate, the material test failure rate is slightly worse than that of Example 1. However, compared with Example 1, Example 2 has the advantages of being able to restrict the setting position of the phase change instrument 10 and simplify the heat resistance treatment.

[0094] Example 3 is the same as Example 1. Figure 1 The results of the continuous annealing equipment of the present disclosure shown in the figure are used to manufacture products from the above materials. However, the heating rate of IH is 5 to 10°C / second. In addition, among the prepared coils, only coils with a thickness of 2 mm are used. Since the heating rate of IH is slow, the length of the IH equipment has to be increased in order to reach the target temperature, resulting in a longer holding time before and after the target temperature is reached. In addition, heating takes time, and the phase change at the condition change point such as the material (target grade) change exceeds expectations and produces an overshoot. Therefore, although it is greatly improved compared to Comparative Example 1, the material test failure rate is slightly worse than that of Example 1.

[0095] Example 4 is the same as Example 1 using Figure 1The results of the continuous annealing equipment of the present disclosure shown in the figure are used to manufacture products from the above materials. However, the heating rate of IH was increased to 200-250°C / second for application. In addition, among the prepared coils, only coils with a plate thickness of 2 mm were used. Since thicker coils were used, even if the heating rate of IH was fast, poor transportation due to deformation did not occur, but due to the high heating rate, overshoot occurred during temperature control. Therefore, although it was greatly improved compared with Comparative Example 1, the material test failure rate was slightly worse than that of Example 1.

[0096] In Example 5, the operating conditions were adjusted by manual control of the operator using the equipment of Example 1 without using automatic feedback control. Since the response was delayed compared to automatic control, the material test failure rate was poor compared to Example 1, but it was significantly improved compared to Comparative Example 1.

[0097] In summary, the continuous annealing equipment, continuous annealing method, cold-rolled steel sheet manufacturing method and plated steel sheet manufacturing method of the present embodiment can respond quickly to the fluctuation of the billet characteristics through the above-mentioned structure, and can minimize the fluctuation (deviation) of the mechanical characteristics of the product. In addition, the continuous annealing equipment of the present embodiment can suppress the length of the production line compared with the past, so it is also expected to reduce the introduction cost of the manufacturing equipment.

[0098] The embodiments of the present disclosure are described based on the drawings and embodiments, but it is to be noted that those skilled in the art can easily make various deformations or modifications based on the present disclosure. Therefore, please note that these deformations or modifications are included in the scope of the present disclosure. For example, the functions included in each component or each step can be reconfigured to avoid logical contradictions, and multiple components or steps can be combined into one, or divided. The embodiments of the present disclosure can also be implemented as a program executed by a processor possessed by the device or a storage medium for recording the program. It should be understood that these are also included in the scope of the present disclosure.

[0099] In the above embodiment, the zinc pot 11 is described as a galvanizing bath for dipping a thin steel sheet, but other plating treatments may be performed. The plating treatment may be, for example, electrogalvanizing, hot-dip galvanizing, or alloy hot-dip galvanizing.

[0100] In addition, for example, the processor of the process computer can read and execute a program stored in the storage unit (e.g., memory) of the process computer to perform processing of predicting the mechanical properties of the final product by the material prediction model, and constructing a phase change rate control model and controlling the output of the induction heating device 9 so that the phase fraction falls within the target range. In addition, the material prediction model and the phase change rate control model can be stored in the storage unit of the process computer.

[0101] Explanation of symbols

[0102] 1 Uncoiler

[0103] 2Welding Machine

[0104] 3Electrolytic cleaning device

[0105] 4. Inlet side looper

[0106] 5. Preheating

[0107] 6 Heating belt

[0108] 7 Tropical

[0109] 8 Cooling belt

[0110] 8A 1st cooling zone

[0111] 8B 2nd cooling zone

[0112] 9Induction heating device

[0113] 10Phase Transformer

[0114] 11 Zinc pot

Claims

1. A continuous annealing equipment, which is a continuous annealing equipment for steel plates, which is composed of a heating zone, a soaking zone and a cooling zone in sequence, and has: at least one induction heating device disposed between the soaking zone and the cooling zone, and At least one measuring device for measuring the phase fraction of the steel plate is provided after the outlet of the induction heating device.

2. The continuous annealing equipment according to claim 1, wherein: A control device is provided for determining an operating condition of the induction heating device based on the measurement result of the measurement device.

3. The continuous annealing equipment according to claim 1 or 2, wherein: The heating speed of the induction heating device is 10°C / second to 200°C / second.

4. A continuous annealing method, wherein a steel plate is sequentially conveyed through a heating zone, a soaking zone and a cooling zone, comprising the following steps: After the soaking zone maintains the temperature of the steel plate in a recrystallization temperature range, the steel plate is rapidly heated by at least one induction heating device disposed between the soaking zone and the cooling zone; and The step of controlling the operating conditions of the induction heating device based on the phase fraction of the steel plate obtained between the outlet of the soaking zone and the inlet of the cooling zone.

5. The continuous annealing method according to claim 4, wherein: The steel plate is heated at 10° C. / sec to 200° C. / sec in the induction heating device.

6. A method for manufacturing a cold-rolled steel sheet, comprising the step of annealing the steel sheet as the cold-rolled steel sheet at an annealing temperature adjusted by the continuous annealing method according to claim 4 or 5.

7. A method for producing a plated steel sheet, comprising the step of: subjecting the surface of the steel sheet annealed by the method for producing a cold-rolled steel sheet according to claim 6 to a plating treatment.

8. The method for producing a plated steel sheet according to claim 7, wherein: The plating treatment is electrogalvanizing, hot-dip galvanizing or alloy hot-dip galvanizing.

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