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

Abstract

Provided are: continuous annealing equipment capable of swiftly responding to a fluctuation in material characteristics and minimizing a fluctuation in mechanical characteristics of a product; a continuous annealing method; a cold-rolled steel sheet manufacturing method; and a plated steel sheet manufacturing method. The continuous annealing equipment is for a steel sheet and is provided with a heating zone (6), a soaking zone (7), and a cooling zone (8) in this order, said continuous annealing equipment comprising: at least one induction heating device (9) which is provided between the soaking zone (7) and the cooling zone (8); and a measurement device (transformation ratio meter 10) which measures at least one steel sheet phase fraction at or after an outlet of the induction heating device (9).

Description

[Technical field]

[0001] The present disclosure relates to 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. The present disclosure particularly relates to 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 for producing a high-strength steel sheet used for automobile structural materials, etc. [Background technology]

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

[0003] In recent years, in the annealing process, a continuous annealing furnace is generally used in which heating, soaking, and cooling are continuously performed while transporting a strip-shaped steel sheet. In addition, after the cooling, hot-dip galvanizing and overaging treatments are performed depending on the application of the steel sheet.

[0004] The heating means for annealing furnaces is generally a radiant tube burner, which uses gas combustion to heat a metal tube (radiant tube) and indirectly heats the steel sheet with the radiant heat. In the manufacture of hot-dip galvanized steel sheets, a direct-fire furnace, which directly injects the flame of gas combustion onto the steel sheet to heat it, is sometimes used in the front stage of the heating furnace to ensure good plating properties.

[0005] Radiant tube furnaces heat steel sheets with radiant heat from the radiant tubes and furnace walls, so the volume of the heat source is very large and the thermal inertia is large. Therefore, it is difficult to quickly follow changes in the set temperature. Furthermore, the temperature rise rate of the steel sheet is slow at the end of the heating stage, and a certain amount of soaking time is required for structure control, so the required furnace length is extended and the thermal inertia is further increased, so that the follow-up to the target temperature is further delayed. As a result, during the continuous processing of coils, the temperature of part of the steel sheet does not fall within the specified annealing temperature range, which can cause problems such as a decrease in yield due to variations in mechanical properties and a decrease in productivity due to changes in line speed for temperature control.

[0006] In response to the above problems, Patent Document 1 discloses a technique for improving responsiveness by supplementing heating capacity by installing an induction heating device between a preheating zone and a heating zone (direct-fired heating furnace) or in the middle of a heating zone consisting of multiple direct-fired heating furnaces. Patent Document 2 discloses a technique for performing highly efficient continuous annealing by installing an electric heating device or an induction heating device at the outlet of the soaking zone. Patent Document 3 discloses a technique for slowly heating from 650°C to 750°C and then rapidly heating from there to a maximum annealing temperature of 750°C to 910°C. Patent Document 4 discloses stabilizing the variation in mechanical properties due to the variation in steel composition and hot rolling conditions by changing the maximum annealing temperature in the longitudinal direction of the steel sheet using a recrystallization and grain growth model. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 11-061277 [Patent Document 2] Japanese Patent Application Publication No. 09-003552 [Patent Document 3] JP 2000-144262 A [Patent Document 4] Japanese Patent Application Publication No. 10-152728 Summary of the Invention [Problem to be solved by the invention]

[0008] However, in the technology of Patent Document 1, in order to prevent excessive oxidation of the steel sheet surface, the steel sheet temperature at the outlet of the direct heating furnace is low, and heating is also required in a radiant tube furnace. This makes it difficult to shorten the furnace length, and the effect of changing the furnace temperature setting is widespread. Furthermore, although it is said that the effect of furnace temperature fluctuations can be absorbed by controlling the output of the induction heating device at high speed, the direct heating furnace following the induction heating device places emphasis on controlling the oxidation of the steel sheet, and it is difficult to flexibly control the steel sheet temperature. Therefore, the effect of absorbing the effect of furnace temperature fluctuations cannot be fully demonstrated.

[0009] The technology of Patent Document 2 targets paint bake hardening (BH) steel sheets, and claims that a high annealing temperature can be achieved without reducing the conveying speed even in a small annealing furnace by installing a heating device at the outlet of the soaking zone. In addition, the technology of Patent Document 2 claims that optimal material can be obtained by introducing a function related to the steel sheet temperature, called the annealing parameter, and making the annealing parameter exceed a threshold value. However, in recent years, with the demand for high strength products, it is necessary to transform α-phase to γ-phase during annealing in Dual Phase steel (DP steel), the production volume of which is increasing. When targeting DP steel, it is necessary to heat the steel sheet in the annealing furnace to a maximum temperature that is equal to or higher than the A1 transformation point and lower than the A3 transformation point, and control the fraction of α-phase and γ-phase by the reached temperature. Although control of the annealing temperature is an important parameter in the production of DP steel, the fraction of α-phase and γ-phase is the primary index in controlling the material. Therefore, the technology of Patent Document 2 does not necessarily stabilize the material of the steel sheet.

[0010] The technology of Patent Document 3 targets interstitial free steel (IF steel), controls the maximum annealing temperature by rapid heating using an induction heating device or electric current heating device before soaking, and absorbs the material variation predicted from the results of the previous process by changing the grain size of α recrystallized grains after annealing. However, when targeting DP steel, if the temperature is rapidly raised to the soaking temperature, i.e., the maximum temperature after slow heating as in the technology of Patent Document 3, the α phase is rapidly transformed into the γ phase while the grain size remains fine, making it impossible to control the phase fraction to the desired value.

[0011] In the technology of Patent Document 4, the steel sheet is heated to the recrystallization region, and then held in that region to allow sufficient recrystallization before being rapidly heated again to the target annealing temperature. This method makes it possible to prevent the rapid transformation from α phase to γ ​​phase caused by rapid heating, and in principle makes it possible to control the phase fraction by controlling the maximum temperature reached. However, in the actual manufacturing process, the transformation behavior varies depending on the chemical composition of the material, so it is difficult to simply control the phase fraction by the steel sheet temperature alone, and it is not possible to suppress the material variation between products.

[0012] In view of the above problems, the present disclosure has an object to provide continuous annealing equipment, a continuous annealing method, a manufacturing method for a cold-rolled steel sheet, and a manufacturing method for a plated steel sheet, which are capable of quickly responding to fluctuations in material properties and minimizing fluctuations in the mechanical properties of products. [Means for solving the problem]

[0013] (1) A continuous annealing facility according to an embodiment of the present disclosure includes: A continuous annealing facility for steel sheets comprising a heating zone, a soaking zone, and a cooling zone in this order, At least one induction heating device provided between the soaking zone and the cooling zone; and a measuring device for measuring at least one phase fraction of the steel sheet, the measuring device being disposed downstream of the outlet of the induction heating device.

[0014] (2) As an embodiment of the present disclosure, in (1), A control device is provided that determines the operating conditions of the induction heating device based on the measurement results of the phase fraction measuring device.

[0015] (3) As an embodiment of the present disclosure, in (1) or (2), The temperature rise rate of the induction heating device is 10° C. / s or more and 200° C. / s or less.

[0016] (4) A continuous annealing method according to an embodiment of the present disclosure, A continuous annealing method for a steel sheet which is transported through a heating zone, a soaking zone, and a cooling zone in this order, comprising: In the soaking zone, the temperature of the steel sheet is maintained in a recrystallization temperature range, and then Rapidly heating the steel sheet by at least one induction heating device provided between the soaking zone and the cooling zone; and controlling the operating conditions of the induction heating device based on the phase fraction of the steel sheet obtained from the outlet of the soaking zone to the inlet of the cooling zone.

[0017] (5) As an embodiment of the present disclosure, in (4), The induction heating device heats the steel plate at a rate of 10° C. / s or more and 200° C. / s or less.

[0018] (6) A method for producing a cold-rolled steel sheet according to an embodiment of the present disclosure includes: The method includes a step of annealing the steel sheet, which is a cold-rolled steel sheet, at an annealing temperature adjusted by the continuous annealing method of (4) or (5).

[0019] (7) A method for producing a plated steel sheet according to an embodiment of the present disclosure includes: (6) A step of plating the surface of the steel sheet annealed by the method for producing a cold-rolled steel sheet.

[0020] (8) As an embodiment of the present disclosure, in (7), The plating treatment is an electrolytic galvanizing treatment, a hot-dip galvanizing treatment, or a hot-dip galvannealing treatment. Effect of the Invention

[0021] According to the present disclosure, there are provided 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 material properties and minimizing fluctuations in the mechanical properties of products. As a result, it becomes possible to manufacture thin steel sheets and the like having target mechanical properties more stably than with conventional annealing furnaces. [Brief description of the drawings]

[0022] [Figure 1] FIG. 1 is a schematic diagram showing a hot-dip galvanizing process equipped with continuous annealing equipment according to one embodiment. [Diagram 2] FIG. 2 is a diagram showing an example of the temperature history of a steel sheet. [Diagram 3] FIG. 3 is a flow chart showing an example of a continuous annealing method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] 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.

[0024] <Equipment configuration> FIG. 1 shows a part of a hot-dip galvanizing process equipped with a continuous annealing facility according to the present embodiment. In this embodiment, the steel material manufactured using the hot-dip galvanizing process is a thin steel sheet. The steel material manufactured is a cold-rolled steel sheet. The arrow in FIG. 1 indicates the line travel direction. Hereinafter, the upstream side in this travel direction may be expressed as "front" and the downstream side as "rear". The continuous annealing facility 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 facility also includes an induction heating device 9 (hereinafter sometimes referred to as "IH") and a transformation rate meter 10. The continuous annealing facility includes at least one induction heating device 9. The continuous annealing facility also includes at least one transformation rate meter 10. 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 zinc plating 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 equipment, an exit looper, a tension reel, and the like.

[0025] The thin steel sheet wound into a coil in the previous process is unwound by a payoff reel 1. The unwound thin steel sheet passes through a preheating zone 5 and then enters a continuous annealing furnace. The continuous annealing equipment is configured to include a heating zone 6, a soaking zone 7, and a cooling zone 8, in this order, 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 after the exit 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 after the exit of the induction heating device 9. Here, the transformation rate meter 10 is an example of a measuring device that measures the phase fraction of a steel sheet.

[0026] Furthermore, the continuous annealing equipment is configured to include a control device that adjusts the output of the induction heating device 9 using the measurement results of the phase fraction by the transformation ratio meter 10. Here, the continuous annealing equipment according to this embodiment is not limited to the configuration in FIG. 1, and can be modified according to 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. In addition, the continuous annealing equipment can be partially divided. For example, the soaking zone 7 may be divided, and each of the divided soaking zones 7 may independently control the atmosphere or temperature.

[0027] <Details of components> FIG. 2 illustrates the temperature history of a steel sheet when annealed in a general annealing furnace of the prior art and when annealed in the annealing furnace of the present embodiment. The vertical axis is the steel sheet temperature. The horizontal axis is the passage time. The line speed is 100 mpm. The thickness of the steel sheet is 1 mm. The temperature history in the present embodiment is shown by a solid line, and the corresponding process (equipment) is written below. The temperature history in the prior art is shown by a dotted line, and the corresponding process (equipment) is written above. As shown in FIG. 2, in the annealing furnace of the present embodiment, the time required for annealing is shorter than that of the prior art due to the configuration described below.

[0028] (Pre-tropical zone) The thin steel sheet discharged from the pay-off reel 1 at a temperature of room temperature to about 100°C first enters the preheating zone 5 and is heated to about 200°C. The temperature of the thin steel sheet may simply increase by heating. In this embodiment, the preheating zone 5 uses a system that utilizes high-temperature exhaust gas generated in the heating zone 6.

[0029] (heating zone) Next, the steel sheet enters the heating zone 6, where it is heated to a temperature of about 600 to 700°C. The temperature of the thin steel sheet may simply increase by heating. In this embodiment, the heating zone 6 employs a direct-fired heating furnace in order to raise the temperature to a certain level in a short time and control the surface condition. The direct-fired heating furnace has high heating capacity and can reduce the volume of the furnace, and also allows for flexible control of the oxidation-reduction reaction on the steel sheet surface, taking into account the subsequent plating process.

[0030] (Solid temperature) The soaking zone 7 mainly plays a role of promoting the recrystallization of the α phase. However, at this time, it does not matter if there are some parts that pass the A1 transformation point. As a heating method for the soaking zone 7, it is preferable to adopt a radiant heating (radiant tube heating) method by gas combustion because of high efficiency and heating uniformity. The soaking zone 7 may be composed of a large single furnace shell. The soaking zone 7 may have two or more compartments, each compartment may be separated by a heat insulating wall or the like, and the steel sheet may be held and heated in a target temperature range determined for each compartment. In this embodiment, the soaking zone 7 holds or slowly heats the steel sheet so that the temperature is in a temperature range (about 600 to 730°C) lower than the A1 transformation point, thereby ensuring the residence time in the recrystallization temperature range and suppressing the remaining of the unrecrystallized α phase. Here, the A1 transformation point is a temperature that causes austenite transformation, and the set furnace temperature is 730°C as an example. In other words, the austenite phase begins to occur at a temperature above the A1 transformation point.

[0031] The recrystallization temperature range is the temperature range in which metallographic recrystallization progresses. In this application, it refers to a temperature range of 600°C or higher and the A1 transformation point or lower. In particular, the specific temperature of the A1 transformation point may be determined from an empirical formula using commonly known alloy component contents, or may be determined in advance by experiments using materials with average alloy components of each product, or may be determined by creating a regression formula based on the alloy component contents from the results of such experiments. The regression formula based on the alloy components may be a commonly known empirical formula.

[0032] 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. On the other hand, if the temperature of the steel sheet in the soaking zone 7 is equal to or higher than the A1 transformation point, the α recrystallized grains become coarse, resulting in insufficient strength. The A1 transformation point may vary slightly depending on the composition of the steel sheet. Therefore, it is preferable to set the recrystallization temperature range by predicting it in advance by measurement, calculation, or simulation, and taking into account the error range during annealing.

[0033] If the residence time of the steel sheet in the soaking zone 7 is too short, the recrystallization of the α phase will not progress sufficiently, and if it is too long, the crystal grains will become coarse and the mechanical properties will deteriorate. Therefore, the optimum residence time was investigated in advance through experiments and it was found that approximately 15 to 80 seconds is necessary. If it is less than 15 seconds, the recrystallization of the α phase will not progress sufficiently and workability will be poor, and if it exceeds 80 seconds, coarse crystal grains will occur in some places and the strength will become non-uniform. Therefore, in the soaking zone 7, the temperature of the steel sheet is held in a temperature range lower than the A1 transformation point for 15 seconds or more and 80 seconds or less.

[0034] When slow heating is performed in the soaking zone 7, it is sufficient that the residence time in the recrystallization temperature range is ensured for the above-mentioned period. However, when slow heating is performed in the soaking zone 7, the heating rate is desirably 5° C. / s or less. This is because if heating is performed rapidly at a rate exceeding 5° C. / s, it becomes impossible to suppress the remaining unrecrystallized α phase.

[0035] In the soaking zone 7, after the residence time in the recrystallization temperature range is ensured, more gradual heating may be performed even if the temperature is equal to or higher than the A1 transformation point, so long as the temperature is in the range of less than 750° C. However, it is preferable that the heating rate is 5° C. / s or less.

[0036] The continuous annealing equipment according to the present embodiment includes a soaking zone 7 using a radiant tube furnace, but may not include such a soaking zone 7 as long as it has a function of ensuring a heat retention time sufficient to advance the recrystallization of the α phase. For example, the soaking zone 7 is not limited to having a radiant tube furnace. The continuous annealing equipment may not include the soaking zone 7, and may have a function of ensuring a heat retention time sufficient for the heating zone 6 to advance the recrystallization of the α phase. The continuous annealing equipment may 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 a portion having the function of the soaking zone 7 (in the above example, a portion or a heat retention device that ensures the heat retention time of the heating zone 6) and the cooling zone 8.

[0037] (Induction heating device) Next, the induction heating device 9 adjusts the output so that the temperature of the steel sheet is within a temperature range (about 750 to 900°C) that is equal to or higher than the A1 transformation point and lower than the A3 transformation point, thereby rapidly heating the steel sheet. Here, the A3 transformation point is the upper limit temperature at which the γ phase fraction can be suppressed. The purpose of this process is to uniformly heat the entire steel sheet to a temperature equal to or higher than the A1 transformation point in a short period of time, which also contributes to the miniaturization of the entire facility. Here, the A1 transformation point and the A3 transformation point may vary slightly depending on the components of the steel sheet. Therefore, it is preferable to set a suitable temperature range by predicting in advance by measurement, calculation or simulation, and taking into account the error range during annealing.

[0038] In the case of DP steel, which is one of the targets of the continuous annealing equipment according to the present embodiment, the temperature reached by the induction heating device 9 has a large effect on the mechanical properties of the final product. Therefore, the heating method of the induction heating device 9 needs to have a fast response to a temperature control command. In addition, when heating in such a temperature range, the Curie point at which the magnetic properties of the steel sheet change is exceeded, so it is desirable for the induction heating device 9 to be of a transverse type.

[0039] In addition, since the soaking zone 7 is maintained in the recrystallization region of the α phase, if the 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 be heated at 10°C / s or more and 200°C / s or less. If the heating rate is less than 10°C / s, the α grains will become coarse, and if the heating rate is more than 200°C / s, local high-temperature areas will occur in the width direction and uniformity will not be maintained. It is more preferable that the induction heating device 9 is heated at 20°C / s or more and 100°C / s or less. If the heating rate is 20°C / s or more, the line length can be shortened, and if the heating rate is 100°C / s or less, the risk of buckling deformation of the steel sheet due to thermal stress can be further reduced.

[0040] When the induction heating device 9 is used to rapidly heat the steel sheet to the target annealing temperature, 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 the target annealing temperature for too long, the transformation from the α phase to the γ phase may proceed more than necessary, making material control more complicated. Therefore, after the steel sheet reaches the target annealing temperature, it is most desirable to enter the steel sheet into 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. In order to avoid a non-equilibrium state, cooling is started within 10 seconds at the latest, even if the steel sheet is held. In other words, cooling of the steel sheet in the cooling zone 8 is started within 10 seconds after the end of heating by the induction heating device 9.

[0041] In order to satisfy such conditions, it is preferable to install the induction heating device 9 at the connection portion between the soaking zone 7 and the cooling zone 8. Providing the induction heating device 9 at the connection portion is also possible for an existing furnace as an additional induction heating device 9.

[0042] (Transformation rate meter) A transformation rate meter 10 for measuring the phase fraction of the steel sheet is disposed on the exit side (after the exit) of the induction heating device 9, and the phase fractions (transformation rates) of the α phase and γ phase of the steel sheet after the target annealing temperature is reached are measured. In particular, it is effective in the present invention to grasp the γ phase fraction with this transformation rate meter 10. Furthermore, the transformation rate meter 10 is provided for the purpose of further appropriately fine-tuning the operation conditions of the induction heating device 9 and the cooling zone 8 that are set in advance based on the γ phase fraction measured by this transformation rate meter 10. By such adjustment, the mechanical properties of the steel sheet can be stably obtained. Here, the installation position of the transformation rate meter 10 is not particularly limited as long as it is after the exit of the induction heating device 9. However, from the viewpoint of feeding back the measured value and using it to control the induction heating device 9 and the cooling zone 8, it is desirable to install it as close to the induction heating device 9 as possible. Specifically, it is preferable to install it between the exit of the induction heating device 9 and the entrance of the cooling zone 8.

[0043] Here, the method of measuring the transformation rate is not particularly limited, but the austenite fraction may be measured using a magnetic detector, i.e., a magnetic transformation rate measuring device consisting of a driving coil that generates a magnetic field and a detection coil that measures the magnetic field that has passed through the steel strip, as a device for measuring the magnetic transformation rate of the steel strip. Specifically, the device described in JP 2019-7907 A can be used. However, in the case of a magnetic transformation rate measuring 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. Therefore, the magnetic transformation rate measuring device may be installed inside the soaking zone 7 or at the exit of the cooling zone 8, rather than 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 may be used. Due to the difference in crystal structure between the γ phase and the α phase, when the steel sheet is irradiated with X-rays, a diffraction peak is generated at each unique angle. The γ phase fraction can be quantified by the intensity of this diffraction peak. For example, X-CAP manufactured by SMS is a commercially available device.

[0044] (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), etc. are used as the cooling means. As in this 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 different cooling means may be combined or the cooling conditions of the same type of cooling means may be changed to control the thermal history of the steel sheet during cooling.

[0045] (Hot-dip galvanizing bath) A hot-dip galvanizing bath is provided following the cooling zone 8, and hot-dip galvanizing can 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 snout, a bath roll, and the like may be provided as necessary.

[0046] (Alloying equipment) The hot-dip galvanizing bath may be followed by an alloying treatment facility. In the alloying treatment facility, the steel sheet is heated and alloyed. The alloying treatment may be carried out according to a conventional method.

[0047] (Other facilities) Following the alloying equipment, a holding zone, a final cooling zone, a temper rolling equipment, a straightener, an outlet looper, a tension reel, etc. may be further provided in order to improve the quality of the final product and to increase the manufacturing stability and efficiency. These facilities may be provided and used according to the quality required for the product, and are not particularly limited.

[0048] <Operation method (control method)> In this embodiment, the optimum range of phase fraction for each product is grasped in advance, and the operating conditions of the annealing furnace are controlled so that the measurement result of the phase fraction by the transformation rate meter 10 falls within the target range. 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 installed downstream of the induction heating device 9 may be adopted as the control object. However, from the viewpoint of responsiveness, the output of the induction heating device 9 is the most effective for stabilizing the material. The control method may be manual control by the operator of the device. Specifically, a monitor that continuously displays the measurement result of the transformation rate meter 10 and the target range of the coil is installed in the operator's cab, and the operating conditions of the above-mentioned control objects are manually changed so that the measurement result falls within the target range. However, since manual control may cause delays in changing conditions or erroneous operation, it is preferable to grasp the optimum range of phase fraction for each product in advance, construct a transformation 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 by the transformation rate meter 10 falls within the target range. This allows the annealing temperature of the steel sheet, which is set in advance, to be appropriately adjusted, making it possible to manufacture products with very stable mechanical properties. A transformation rate control model is constructed for each product, and in this embodiment, the strength grade is used as a factor for classifying products. The factor for classifying products is not limited to the strength grade. For example, products may be classified according to required product properties, such as the steel type, mechanical properties other than strength, and surface properties, and a transformation rate control model for each product may be constructed.

[0049] FIG. 3 is a flowchart showing an example of a continuous annealing method carried out in the above-mentioned continuous annealing facility in this embodiment.

[0050] The mechanical properties (mechanical properties) of the final product are predicted using a material prediction model based on the phase fraction measurement results using a transformation rate meter 10 in the annealing furnace ("Transformation rate information results" in FIG. 3), the chemical composition information obtained when the material was cast ("Material information" in FIG. 3), and the current operating conditions of the continuous annealing equipment (particularly the operating conditions of the cooling zone 8). At this time, the material prediction model used may be not only a physical model obtained by offline laboratory experiments or numerical analysis, but also a machine learning model obtained from accumulated manufacturing results.

[0051] Next, it is determined whether the predicted mechanical properties ("Prediction result of mechanical properties" in FIG. 3) are within the target range of mechanical properties allowed for the product. The target range of mechanical properties is given by a host computer. The host computer is a computer that gives manufacturing information, etc. to a process computer that manages the operation of the continuous annealing equipment. If the properties are within the target range ("Within target range of mechanical properties" in FIG. 3 is Yes), the continuous annealing equipment continues operation under the current conditions. If the properties are outside the target range ("Within target range of mechanical properties" in FIG. 3 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 described above, and to determine the steel sheet temperature control amount to realize the phase fraction 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.

[0052] When the continuous annealing equipment determines that the required temperature control range is included in the range that can be controlled by the induction heating equipment 9 ("IH output control range" in FIG. 3 is Yes), it changes the output of the induction heating equipment 9. When the continuous annealing equipment determines that the required temperature control range is not included in the range that can be controlled by the induction heating equipment 9 ("IH output control range" in FIG. 3 is No), it also changes the operating conditions of the equipment other than the induction heating equipment 9 ("Change operating conditions of other equipment" in FIG. 3). The controlled object is the equipment installed upstream of the transformation rate meter 10, for example, the cooling zone 8 or the soaking zone 7. The controlled item may be the injection pressure of the cooling gas when gas jet cooling is adopted in the cooling zone 8, or the furnace temperature when the soaking zone 7 is adopted. These equipments have a lower responsiveness to changes in operating conditions than the induction heating equipment 9, but by controlling them in combination with the induction heating equipment 9 via the above control flow, it is possible to minimize the amount by which the mechanical properties of the product deviate from the target.

[0053] Here, even in the conventional continuous annealing equipment, it is possible to install a transformation ratio meter 10 and attempt to control the transformation ratio by highly accurate control of the sheet temperature. However, in the equipment specifications of the conventional radiant heating type annealing furnace only, it takes a long time to heat (for example, about 100 to 200 seconds), so the responsiveness of the furnace temperature to the control command is very poor. In addition, the steel sheet length affected by the change in furnace temperature is also long, so the steel sheet temperature cannot be finely controlled. On the other hand, in the continuous annealing equipment according to the present embodiment, the steel sheet temperature can be controlled at high speed (for example, 10 or less, about 5 seconds). In addition, since it is not necessary to complete the transformation to the γ phase in the soaking zone 7, the furnace length can be shortened (for example, 70 to 110 m), so the responsiveness to the command to change the furnace temperature due to the change in the steel sheet size or the change in the steel type is high. As a result, it is possible to quickly reflect the change in the heating conditions not only in response to the transformation behavior fluctuation caused by the material, but also when the annealing conditions change at the product joint. In addition, the range that deviates from the target annealing conditions can be reduced, and the yield is significantly improved.

[0054] The steel sheet may be subjected to annealing in a continuous annealing facility, followed by plating, alloying, temper rolling, and shape correction. The plating and alloying treatments may be conventional methods that satisfy the quality required for the surface properties of the product, and are not particularly limited.

[0055] In addition, if the shape is disturbed, the steel sheet may be subsequently subjected to temper rolling and straightening by passing through a straightening machine. The temper rolling and straightening may be performed under conditions that merely straighten the shape without affecting the mechanical properties of the steel sheet, and are not limited thereto.

[0056] (Target material) The continuous annealing equipment according to this embodiment anneals steel materials including DP steels to stably obtain the target mechanical properties. Specific examples of the steel sheets to be treated include those containing, by weight%, C 0.03% or more and 0.25% or less, Si 0.01% or more and 2.50% or less, Mn 0.50% or more and 4.00% or less, P 0.100% or less, S 0.0500% or less, Sol.Al 0.005% or more and 2.000% or less, and N 0.100% or less, 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 inevitable 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.

[0057] (Example) Table 1 shows the conditions and results of producing thin steel sheets using a conventional continuous annealing facility and a continuous annealing facility according to the present embodiment (see FIG. 1). Here, except for Comparative Example 4, in order to investigate the variation in the mechanical properties of the products, 30 products of each strength level were produced in coils. Three types of strength levels, 780 MPa, 980 MPa, and 1180 MPa, were produced, and three types of plate thicknesses, 1.0, 1.5, and 2.0 mm, were produced at each strength level, with 10 slabs each. These 10 slabs of each grade and plate thickness were all cast in different lots by a continuous casting machine. Therefore, although within the manufacturing control range, the chemical composition of each slab varies and the transformation behavior is not uniform. In Table 1, the equipment configuration shows the devices that constitute the equipment. Also, in Table 1, FB indicates feedback and FF indicates feedforward.

[0058] [Table 1]

[0059] Each slab was hot-rolled and pickled by a general method, and annealed and cold-rolled as necessary, and then heat-treated in a laboratory, a conventional continuous annealing facility, and the continuous annealing facility of the present disclosure, followed by post-treatment such as cooling and plating. Tensile test pieces were taken every 100 m in the steel sheet conveying direction of the final product coil, and mechanical properties were measured. Here, the tensile test pieces were JIS No. 5. The tensile test was performed according to JIS Z 2241. If the tensile test result for the entire coil length was outside the lower limit of the target value, the length 10 m before and after the test piece was taken as the material test failure length. The ratio of this material test failure length to the entire coil length was obtained as the "material test (abbreviation for material test) failure rate". In addition, the required strength ranges for each grade of 780 MPa, 980 MPa, and 1180 MPa are 780 MPa or more, 980 MPa or more, and 1180 MPa or more, respectively.

[0060] Comparative Example 1 was manufactured from the above-mentioned material using a conventional annealing furnace that was not equipped with an induction heating device 9 and a transformation rate meter 10 and was composed of a preheating zone 5, a heating zone 6, and a soaking zone 7. The line speed in the manufacturing process was in the range of 60 to 120 mpm, and the heat pattern during steel sheet annealing was controlled according to the sheet thickness and components. The steel sheet temperature at the exit of the heating zone 6 (direct flame heating) was set in the range of 600 to 700°C to control the oxidation-reduction reaction on the surface. The steel sheet was then introduced into the soaking zone 7 of the radiant tube type for further heating, and the residence time in the recrystallization zone (about 600 to 730°C) was 15 to 80 seconds. The heating / soaking time after the recrystallization zone was 100 to 200 seconds. The steel sheet temperature at the exit of the soaking zone 7 (entrance to the cooling zone 8) was in the range of 750 to 850°C. The furnace temperature of the soaking zone 7 was controlled so that the steel sheet temperature at the exit was a target value. After that, several 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 of each grade varied greatly, with some below the lower limit for each grade, and many parts did not meet the material properties. The cause of this is presumed to be that the phase fraction at the time of annealing was not the targeted value due to fluctuations in the chemical composition of the material, and when the annealing conditions changed at the joint of coils with different plate thicknesses and grades, there were parts where the heating conditions could not keep up.

[0061] In Comparative Example 2, the same conventional annealing furnace as in Comparative Example 1 was used, and a transformation rate meter was installed at the outlet of the soaking zone 7 to manufacture a final product coil. The basic manufacturing conditions were set to be the same as in Comparative Example 1, but the transformation rate was measured using the transformation rate meter 10 at the outlet of the soaking zone 7, and the furnace temperature of the soaking zone 7 was controlled. Since the furnace temperature of the soaking zone 7 was changed to reflect the obtained transformation rate, the temperature fluctuation range at the inlet of the cooling zone 8 became larger than in Comparative Example 1, but since the furnace temperature of the soaking zone 7 was actively controlled, the failure rate of the material test was improved compared to Comparative Example 1. However, it is presumed that there were some parts that the control of the annealing furnace could not follow, such as the joints of coils with different plate thicknesses and the joints of coils with different annealing conditions, and the failure of the material test could not be suppressed.

[0062] In Comparative Example 3, the same conventional annealing furnace as in Comparative Example 1 was used, and a transformation rate meter was installed at the exit of the cooling zone 8 to manufacture the final product coil. The basic manufacturing conditions were set to be the same as in Comparative Example 1, but the transformation rate was measured by the transformation rate meter 10 at the exit of the cooling zone 8, and the furnace temperature of the soaking zone 7 and the coolant injection pressure of the cooling zone 8 were controlled. Since the furnace temperature of the soaking zone 7 was changed to reflect the obtained transformation rate, the temperature fluctuation range at the entrance of the cooling zone 8 became larger than in Comparative Example 1, but the furnace temperature control of the soaking zone 7 was actively performed, so the failure rate of the material test was improved compared to Comparative Example 1. In addition, the coolant injection pressure of the cooling zone 8 was also controlled, so the failure rate of the material test was improved compared to Comparative Example 2. However, it is estimated that there were some parts that the control of the annealing furnace could not follow, such as the joints of coils with different plate thicknesses and the joints of coils with different annealing conditions, and the failure of the material test could not be suppressed.

[0063] In Comparative Example 4, induction heating equipment was provided at the exit of the soaking zone 7 of a conventional annealing furnace. The sheet passing speed and the steel sheet temperature at the exit of the heating zone 6 were controlled within the same range as in Comparative Example 1. Although the induction heating device 9 was provided, the measured value of the transformation rate meter 10 was not fed back to the output control and furnace temperature control of the induction heating device 9, and control was performed only by the steel sheet temperature measured in the furnace or calculated as in the conventional case. As a result, the controllability of the steel sheet temperature was improved and the variation in the mechanical properties of the product was improved, but the variation due to the chemical composition of the material could not be absorbed, and the material test failure rate did not improve significantly from Comparative Example 1.

[0064] In Comparative Example 5, an induction heating device was provided at the entrance of the soaking zone 7 of a conventional annealing furnace. A transformation rate meter 10 was also provided at the exit of the soaking zone 7. The sheet threading speed and the steel sheet temperature at the exit of the heating zone 6 were controlled within the same range as in Comparative Example 1. An operator manually controlled the operating conditions of each piece of equipment in the annealing furnace based on the measurement results of the transformation rate meter 10. Although the induction heating device 9 was provided, the residence time in the recrystallization zone was very short, and the transformation from the α phase to the γ phase progressed rapidly without sufficient recrystallization. Therefore, even when the transformation rate was measured at the exit of the soaking zone 7, it did not change to a single γ phase, and the structure could not be controlled as intended.

[0065] Comparative Example 6 is an example in which an automatic feedback control system was applied to control the operating conditions of the soaking zone 7 and the induction heating device 9 based on the measurement results of the transformation rate meter 10, as compared to Comparative Example 5. Although the control method was changed, it was not possible to control the transformation from the α phase to the γ phase, as in Comparative Example 5, and it was not possible to obtain the targeted structure.

[0066] In Comparative Example 7, a transformation rate meter 10 was provided at the exit of the soaking zone 7 of a conventional annealing furnace, and an induction heating device 9 was provided downstream of the transformation rate meter 10. A feedforward control system was adopted that controls the operating conditions of the downstream induction heating device 9 based on the phase fraction information measured by the transformation rate meter 10. However, since most of the steel remained in the α single phase at the exit of the soaking zone 7, the output of the induction heating device 9 remained almost constant during production, and therefore the fluctuation in mechanical properties was not improved.

[0067] Example 1 shows the results of manufacturing a product from the above-mentioned material using the continuous annealing equipment of the present disclosure shown in Fig. 1. The sheet threading speed and the steel sheet temperature at the outlet of the heating zone 6 were controlled within the same range as in Comparative Example 1. However, in Example 1, the phase fraction immediately after annealing was measured using a transformation rate 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 were controlled. In this way, by controlling based on the phase fraction, the mechanical properties of the product were stable, and no material test defects occurred over the entire length.

[0068] In Example 2, similarly to Example 1, a product was manufactured from the above-mentioned material using the continuous annealing equipment of the present disclosure shown in FIG. 1. However, the installation position of the transformation rate meter 10 was changed to the exit of the cooling zone 8. The distance between the transformation rate meter 10, which is a sensor, and the induction heating device 9, which is the actuator used preferentially, was large, so there was a delay in response compared to Example 1. Although there was a certain effect in improving the material test failure rate, the material test failure rate was slightly inferior to Example 1, because there were some locations where the output change of the induction heating device 9 did not follow. However, compared to Example 1, Example 2 has the advantage of being able to restrict the installation position of the transformation rate meter 10 and simplify the heat resistance treatment.

[0069] In Example 3, similarly to Example 1, a product was manufactured from the above-mentioned material using the continuous annealing equipment of the present disclosure shown in FIG. 1. However, the IH heating rate was 5 to 10°C / s. In addition, only coils with a plate thickness of 2 mm were used among the prepared coils. Because the IH heating rate was slow, the IH equipment length had to be increased in order to reach the target temperature, and as a result, the holding time around the target temperature was long. In addition, heating took a long time, and at the point of change in conditions such as a change in the material (target grade), the transformation progressed more than expected, causing an overshoot. Therefore, although there was a significant improvement compared to Comparative Example 1, the material test failure rate was slightly inferior to Example 1.

[0070] In Example 4, similar to Example 1, a product was manufactured from the above-mentioned material using the continuous annealing equipment of the present disclosure shown in FIG. 1. However, the IH heating rate was increased to 200 to 250 ° C. / s. Of the prepared coils, only those with a plate thickness of 2 mm were used. Since a thick coil was used, no transportation failure due to deformation occurred even when the IH heating rate was fast, but an overshoot occurred during temperature control due to the high heating rate. Therefore, although there was a significant improvement compared to Comparative Example 1, the material test failure rate was slightly inferior to Example 1.

[0071] In Example 5, the automatic feedback control was not used in the equipment of Example 1, and the operating conditions were adjusted by manual control by the operator. Since there was a delay in response compared to automatic control, the material test failure rate was inferior to that of Example 1, but a large improvement was observed compared to Comparative Example 1.

[0072] As described above, the continuous annealing equipment, the continuous annealing method, the manufacturing method of a cold-rolled steel sheet, and the manufacturing method of a plated steel sheet according to the present embodiment can quickly respond to fluctuations in material properties and minimize fluctuations (variations) in the mechanical properties of the product due to the above-mentioned configurations. Furthermore, since the continuous annealing equipment according to the present embodiment can reduce the line length compared to conventional methods, a reduction in the introduction cost of the manufacturing equipment can also be expected.

[0073] 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 can easily make various modifications or corrections based on the present disclosure. Therefore, it should be noted that these modifications or corrections are included in the scope of the present disclosure. For example, the functions included in each component or each step can be rearranged so as not to be logically inconsistent, and multiple components or steps can be combined into one or divided. The embodiments of the present disclosure can also be realized as a program executed by a processor included in the device or a storage medium on which a program is recorded. It should be understood that these are also included in the scope of the present disclosure.

[0074] Although in the above embodiment the zinc pot 11 has been described as a galvanizing bath in which the steel sheet is immersed, other galvanizing processes may be carried out, for example an electrogalvanizing process, a hot-dip galvanizing process or a hot-dip galvannealing process.

[0075] Furthermore, for example, a program stored in a storage unit (e.g., a memory) of the process computer may be read and executed by a processor of the process computer to perform a process of predicting mechanical properties of a product finally obtained from a material prediction model, a process of constructing a transformation rate control model, and a process of controlling the output of the induction heating device 9 so that the phase fraction falls within a target range, etc. Furthermore, the material prediction model and the transformation rate control model may be stored in the storage unit of the process computer. [Explanation of symbols]

[0076] 1 Payoff Reel 2. Welding machine 3. Electrolytic cleaning equipment 4 Inlet looper 5 Pre-tropical zone 6 Heating Zone 7. Equal temperature 8 Cooling Zone 8A First Cooling Zone 8B Second Cooling Zone 9 Induction heating device 10 Transformation Rate Meter 11 Zinc Pot

Claims

1. A continuous annealing facility for steel sheets comprising a heating zone, a soaking zone, and a cooling zone in this order, at least one induction heating device disposed between the soaking zone and the cooling zone; A measuring device for measuring at least one phase fraction of the steel sheet, the measuring device being disposed downstream of the outlet of the induction heating device; A control device that determines operating conditions of the induction heating device based on the measurement results of the measurement device, The control device includes: In the soaking zone, the temperature of the steel sheet is maintained in a recrystallization temperature range, and then The steel sheet is rapidly heated by the induction heating device, A continuous annealing facility that controls operating conditions of the induction heating device based on a phase fraction of the steel sheet obtained downstream of the induction heating device between an outlet of a soaking zone and an inlet of a cooling zone.

2. 2. The continuous annealing facility according to claim 1, wherein the induction heating device has a temperature rise rate of 10° C. / s or more and 200° C. / s or less.

3. A continuous annealing method for a steel sheet which is transported through a heating zone, a soaking zone, and a cooling zone in this order, comprising: In the soaking zone, the temperature of the steel sheet is maintained in a recrystallization temperature range, and then Rapidly heating the steel sheet with at least one induction heating device disposed between the soaking zone and the cooling zone; and controlling operating conditions of the induction heating device based on a phase fraction of the steel sheet obtained downstream of the induction heating device between an outlet of the soaking zone and an inlet of a cooling zone.

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

5. A method for producing a cold-rolled steel sheet, comprising a step of annealing the steel sheet, which is a cold-rolled steel sheet, at an annealing temperature adjusted by the continuous annealing method according to claim 3 or 4.

6. 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 5.

7. The method for producing a plated steel sheet according to claim 6, wherein the plating treatment is an electrolytic galvanizing treatment, a hot-dip galvanizing treatment, or a hot-dip galvannealing treatment.

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