JP2019173169A - Facility for continuous annealing of steel sheet and method for manufacturing annealed steel sheet - Google Patents

Abstract

An object of the present invention is to provide a steel sheet continuous annealing equipment capable of producing various kinds of high-strength steel sheets and plated steel sheets according to a wide variety of individual requirements with one equipment. The steel sheet continuous annealing equipment of the present invention comprises a heating zone for heating the steel sheet, a slow cooling zone for gradually cooling the steel sheet passing through the heating zone, and a cooling zone for cooling the steel sheet passing through the slow cooling zone. And a reheating zone for reheating the steel sheet that has passed through the cooling zone, wherein the steel sheet that has passed through the cooling zone is gradually cooled between the cooling zone and the reheating zone. Alternatively, it has a slow cooling zone for keeping the temperature warm, and a recooling zone for cooling again the steel sheet passing through the slow cooling zone. [Selection diagram] Fig. 1

Description

  The present invention relates to a steel plate continuous annealing facility and a method for manufacturing an annealed steel plate.

  In recent years, from the viewpoint of achieving both improvement in automobile fuel efficiency and improvement in collision safety, automobile bodies are required to be lighter and stronger. For this reason, a high-strength steel sheet having high tensile strength and excellent deep drawability and being thinned is used as a vehicle body material. Further, for example, in parts that require particularly high collision safety, in addition to increasing strength and reducing wall thickness, a balance between strength and ductility, a high yield ratio, and excellent hole expandability are required. Thus, even if it is a high-strength steel plate, the desired characteristic changes with places used.

  As a method for producing a high-strength steel sheet having such desired characteristics, a method using a rapid cooling technique is known (see, for example, JP-A-2015-38234). In this conventional method for producing a high-strength steel sheet, when the steel sheet is continuously annealed, the heated steel sheet is quenched, and the quenched steel sheet is heated to maintain the surface temperature of the steel sheet at a constant temperature. Quench quickly. By using this conventional method for producing a high-strength steel sheet, it is said that a steel strip having desired characteristics can be stably produced without reducing productivity and quality.

  In a conventional method for producing a high-strength steel sheet, a steel sheet continuous annealing facility having a rapid cooling device by water cooling is used as the rapid cooling technique. However, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets that are excellent in rust prevention are sometimes used as high-strength steel sheets. However, when water-cooled, an oxide film is formed on the surface, making it difficult to plate the steel sheets. Become. For this reason, when manufacturing a high intensity | strength plated steel plate, the other steel plate continuous annealing equipment which does not have the rapid cooling apparatus by water cooling is needed.

  For this reason, there is a need for a steel plate continuous annealing facility that can produce a wide variety of high-strength steel plates and plated steel plates that meet a wide variety of individual requirements with a single facility.

JP-A-2015-38234

  The present invention has been made based on the circumstances as described above, and an object thereof is to provide a steel plate continuous annealing facility capable of manufacturing various types of high-strength steel plates and plated steel plates according to various requirements with one facility. And

  The present invention made to solve the above problems includes a heating zone for heating a steel plate, a slow cooling zone for gradually cooling the steel plate that has passed through the heating zone, and a cooling zone for cooling the steel plate that has passed through the slow cooling zone. And a re-heating zone for reheating the steel plate that has passed through the cooling zone, and the steel plate that has passed through the cooling zone is gradually cooled between the cooling zone and the re-heating zone. Or it has the slow cooling warming zone which heat-retains, and the re-cooling zone which cools again the steel plate which passed the said slow cooling warming zone.

  Since the said steel plate continuous annealing equipment is equipped with a heating zone, a slow cooling zone, a cooling zone, and a reheating zone in this order, it can manufacture many types of steel plates from a soft steel plate to a high strength steel plate. Moreover, the said steel plate continuous annealing equipment equips a cooling zone and a reheating zone in this order with a slow cooling heat retention zone and a recooling zone. By performing this heat treatment, a steel structure in which bainite, martensite, retained austenite, and unstable austenite are mixed can be obtained. The steel sheet continuous annealing equipment can adjust the balance of tensile strength, ductility, yield ratio, hole expandability, deep drawability, and the like at a high level by controlling the mixing ratio of these. Furthermore, in the said steel plate continuous annealing equipment, since it is not necessary to perform rapid cooling by water cooling, a plated steel plate can also be manufactured. Therefore, by using the steel plate continuous annealing equipment, it is possible to produce a wide variety of high-strength steel sheets and plated steel sheets according to various requirements with one equipment.

  The slow cooling zone, the cooling zone, and the recooling zone may include a gas jet cooling device as a cooling means for the steel plate. The gas jet cooling device can perform cooling using a mixed gas of nitrogen and hydrogen. For this reason, it can cool by suppressing the oxidation of the steel plate surface by making the said cooling means into a gas jet cooling device. Therefore, it can suppress that non-plating generate | occur | produces when manufacturing a plated steel plate.

  The slow cooling and warming zone may include an induction heater or an indirect heating type heater as a heating means for making the cooling rate of the steel sheet weaker than the cooling rate of the cooling zone. Thus, by using an induction heater or an indirect heating type heater as the heating means, the steel sheet can be gradually cooled or kept warm while suppressing oxidation of the steel sheet surface. Therefore, it can suppress that non-plating generate | occur | produces when manufacturing a plated steel plate.

  The length of the slow cooling and warming zone in the transport direction is preferably 3 to 8 times the length of the cooling zone in the transport direction. By making the length of the slow cooling and warming zone in the transport direction within the above range, the amount of bainite formed in the slow cooling and warming zone is controlled to an appropriate amount, so that a steel sheet having an excellent balance between strength and ductility is obtained. can get.

  The length of the slow cooling and warming zone in the transport direction is preferably 0.7 to 2 times the length of the slow cooling zone in the transport direction. By making the length of the slow cooling and warming zone in the transport direction within the above range, the amount of bainite formed in the slow cooling and warming zone is controlled to an appropriate amount, so that a steel sheet having an excellent balance between strength and ductility is obtained. can get.

  The length in the transport direction of the slow cooling and warming zone is preferably 30 m or more. Thus, by making the length of the slow cooling and warming zone in the transport direction equal to or more than the above lower limit, it is possible to improve the transport speed of the steel sheet while securing the amount of bainite formed in the slow cooling and warming zone. Can be increased.

Another method of manufacturing an annealed steel sheet according to the present invention, which has been made to solve the above problems, includes a step of heating a steel sheet, a step of gradually cooling the steel sheet after the heating step, and a cooling of the steel sheet after the annealing step. A step of slowly cooling and keeping the steel plate after the cooling step, a step of recooling the steel plate after the slow cooling and keeping step, and a step of reheating the steel plate after the recooling step, Using the steel plate continuous annealing equipment of the invention, the atmosphere of the heating zone in the heating step contains hydrogen and water vapor, the temperature of the steel plate surface in the heating step is T (K), and the dew point in the heating zone is DP ( K) and a gas constant of R (= 8.3144598JK ⁻¹ mol ⁻¹ ), the dew point of the atmosphere during heating in the heating step is −15 ° C. or more and 15 ° C. or less, and the hydrogen concentration H of the atmosphere is H (Vol%) is made to satisfy | fill following formula (1).

  The manufacturing method of the said annealed steel plate can manufacture many types of high-strength steel plates and plated steel plates according to various individual requirements with one facility by using the steel plate continuous annealing facility of the present invention. Further, in the heating step, the dew point is set within the above range and the hydrogen concentration is heated so as to satisfy the above formula (1), so that the iron foil hardly adheres to the steel plate or the equipment surface and the wall surface in the heating zone within the heating zone. The occurrence of poor quality of steel sheets can be suppressed.

  Here, “slow cooling” refers to gradual cooling at a cooling rate of 20 ° C./s or less, and “cooling” refers to cooling faster than gradual cooling at a cooling rate of greater than 20 ° C./s. . In addition, the “length in the conveying direction” means that in the slow cooling zone, the cooling zone, and the recooling zone, an area where forced cooling is performed, for example, a gas jet cooling device is used. Indicates the length of the steel plate passing through the area where the steel plate is sprayed. In the slow cooling and warming zone, it indicates the length of the zone from the outlet of the zone of the cooling zone to the inlet of the zone of the recooling zone. In each zone, the steel sheet may be fed while being folded by a roll. In such a case, the “length in the conveyance direction” is the total conveyance length including the folding.

  As described above, the steel sheet continuous annealing equipment of the present invention can produce a wide variety of high-strength steel sheets and plated steel sheets according to various requirements with one equipment.

It is a schematic diagram which shows the structure of the steel plate continuous annealing equipment which concerns on one Embodiment of this invention.

  Hereinafter, the steel plate continuous annealing equipment and the method of manufacturing the annealed steel plate according to the present invention will be described with reference to the drawings as appropriate.

[Steel sheet continuous annealing equipment]
The steel plate continuous annealing facility shown in FIG. 1 cools the heating zone 1 that heats the steel plate M, the slow cooling zone 2 that gradually cools the steel plate M that has passed the heating zone 1, and the steel plate M that has passed the slow cooling zone 2. It is a continuous annealing facility for a steel plate M having a cooling zone 3 and a reheating zone 4 for reheating the steel plate M that has passed through the cooling zone 3. Moreover, the said steel plate continuous annealing equipment passed between the cooling zone 3 and the reheating zone 4, the slow cooling and warming zone 5 that gradually cools or keeps the steel plate M that passed through the cooling zone 3, and the slow cooling and warming zone 5. And a re-cooling zone 6 for cooling the steel plate M again. Furthermore, the said steel plate continuous annealing equipment has the soaking zone 7 which maintains the temperature of the steel plate M which passed the heating zone 1 between the heating zone 1 and the slow cooling zone 2. FIG.

<Steel plate>
The steel plate M charged in the steel plate continuous annealing equipment is a thin steel plate manufactured by, for example, cold rolling after hot rolling, and the average thickness of the steel plate M is, for example, 0.5 mm or more and 2.5 mm It is as follows.

  The steel plate M contains an appropriate amount of elements such as C, Si, Mn, P, S, and Al according to required characteristics. The balance of the steel plate M is iron and inevitable impurities.

  In the steel plate continuous annealing facility, the steel plate M is transported in the order of the heating zone 1, the soaking zone 7, the slow cooling zone 2, the cooling zone 3, the slow cooling and warming zone 5, the recooling zone 6, and the reheating zone 4. In addition, the white arrow of FIG. 1 has shown the conveyance direction of the steel plate M. FIG.

  Although the conveyance speed of the steel plate M is not specifically limited, As a minimum of the conveyance speed of the steel plate M, 20 mpm (m / min) is preferable and 30 mpm is more preferable. On the other hand, as an upper limit of the conveyance speed of the steel plate M, 100 mpm is preferable and 60 mpm is more preferable. There exists a possibility that manufacturing efficiency may fall that the conveyance speed of the steel plate M is less than the said minimum. On the contrary, when the conveyance speed of the steel sheet M exceeds the above upper limit, for example, in the processing zone in which the cooling rate such as the slow cooling zone 2 is defined, it is necessary to increase the length of the processing zone in order to complete the necessary processing. The equipment and running costs may increase.

  Hereinafter, each processing zone will be described in the order of passage of the steel plate M.

<Heating zone>
In the heating zone 1, the steel plate M is heated.

  Although it does not specifically limit as a heating means of the heating zone 1, An induction heater, a heating means of a direct fire heating system, etc. can be used.

  Although the average heating rate of the steel plate M in the heating zone 1 is not particularly limited, it can be, for example, 1 ° C./s or more and 20 ° C./s or less. When the average heating rate is less than the lower limit, it takes time to reach a desired temperature, and thus production efficiency may be reduced. Conversely, if the average heating rate exceeds the upper limit, the equipment for heating and running costs may increase.

  The heating temperature (heating end temperature) of the steel sheet M in the heating zone 1 is appropriately determined so that the steel sheet M having desired characteristics is obtained after annealing, and is usually higher than 650 ° C. The heating temperature is preferably a temperature of Ac1 point × 0.8 + Ac3 point × 0.2 or more and less than Ac3 point. If the heating temperature is less than the lower limit, the strength and ductility of the steel sheet M after annealing may not be balanced. Conversely, if the heating temperature is equal to or higher than the upper limit, the ductility of the steel sheet M after annealing may be reduced. The Ac1 point is a temperature at which the structure of the steel sheet M starts to transform from ferrite + cementite to austenite by heating, and the Ac3 point is a temperature at which the structure of the steel sheet M completes the transformation from ferrite to austenite by heating.

<Soaking>
In the soaking zone 7, the temperature of the steel sheet M that has passed through the heating zone 1 is maintained.

  The heated steel sheet M is at a high temperature, and it is preferable to perform heating in order to maintain the temperature. As the heating means, the same means as the heating zone 1 may be used.

  The time (processing time) for the steel sheet M to pass through the soaking zone 7 is preferably 1 second or more and 1800 seconds or less. If the treatment time is less than the lower limit, the transformation of the structure of the steel plate M may not be completed sufficiently. Conversely, if the treatment time exceeds the upper limit, the production efficiency may be reduced.

<Slow cooling zone>
In the slow cooling zone 2, the steel sheet M that has passed through the soaking zone 7 is slowly cooled.

  The slow cooling zone 2 preferably includes a gas jet cooling device 2a as a means for cooling the steel plate M as shown in FIG. The gas jet cooling device 2a can perform cooling using a mixed gas of nitrogen and hydrogen. For this reason, by making the said cooling means into the gas jet cooling device 2a, slow cooling can be performed, suppressing the oxidation of the steel plate M surface. Therefore, it can suppress that non-plating generate | occur | produces when manufacturing a plated steel plate.

  The average cooling rate of the steel sheet M in the slow cooling zone 2 is not particularly limited as long as it is slow cooling (cooling rate of 20 ° C./s or less), but is, for example, 0.1 ° C./s or more and 10 ° C./s or less. . If the average cooling rate is less than the lower limit, the production efficiency may be reduced, or the time during which the steel plate M is exposed to high-temperature residual heat becomes long, and the transformation of the structure of the steel plate M may be excessively advanced. Conversely, if the average cooling rate exceeds the upper limit, the steel sheet M may be subjected to thermal distortion due to cooling.

  The cooling temperature of the steel sheet M in the slow cooling zone 2 (end temperature of slow cooling) is preferably 650 ° C. or higher and 800 ° C. or lower. When the cooling temperature is less than the lower limit, the cooling rate is slow in the slow cooling zone 2, so that the time during which the steel plate M is exposed to high temperature residual heat becomes long, and the transformation of the structure of the steel plate M may proceed excessively. . Conversely, when the cooling temperature exceeds the upper limit, there is a risk that thermal distortion due to cooling occurs in the steel sheet M in the cooling zone 3 following the slow cooling zone 2.

<Cooling zone>
The cooling zone 3 cools the steel plate M that has passed through the slow cooling zone 2.

  The cooling zone 3 preferably includes a gas jet cooling device 3a as a cooling means for the steel plate M as shown in FIG. The gas jet cooling device 3a can perform cooling using a mixed gas of nitrogen and hydrogen. For this reason, by using the gas jet cooling device 3a as the cooling means, cooling can be performed while suppressing oxidation of the surface of the steel sheet M. Therefore, it can suppress that non-plating generate | occur | produces when manufacturing a plated steel plate.

  The average cooling rate of the steel sheet M in the cooling zone 3 is preferably 30 ° C./s or more and less than 200 ° C./s. If the average cooling rate is less than the lower limit, excessive ferrite may be formed on the steel sheet M. Conversely, if the average cooling rate is equal to or higher than the upper limit, the steel sheet M may be subject to thermal distortion due to cooling. In the cooling zone 3, it is preferable to rapidly cool the steel sheet M, and the average cooling rate may be determined so that the cooling time to the desired temperature is 30 seconds or less, preferably 20 seconds or less.

  As a minimum of the cooling temperature (cooling end temperature) of steel plate M in cooling zone 3, 300 ° C is preferred and 340 ° C is more preferred. On the other hand, the upper limit of the cooling temperature is preferably 500 ° C and more preferably 460 ° C. If the cooling temperature is less than the lower limit, the carbon concentration region in the steel sheet M becomes small, and coarse retained austenite cannot be secured, so that the deep drawability of the steel sheet M after annealing may be reduced. On the contrary, when the cooling temperature exceeds the upper limit, the carbon concentration region in the steel sheet M becomes large, and the retained austenite and the composite structure (MA) of martensite and austenite become too coarse. May decrease.

<Slow cooling and warming zone>
In the slow cooling and keeping zone 5, the steel sheet M that has passed through the cooling zone 3 is slowly cooled.

  In the slow cooling and keeping zone 5, it is necessary to control the cooling rate of the steel sheet M rapidly cooled in the cooling zone 3 to a cooling rate of 20 ° C./s or less. For this reason, in the slow cooling heat retention zone 5, it is preferable to heat the steel plate M at least in the first half to reduce the cooling rate. The slow cooling and keeping zone 5 is preferably provided with an induction heater 5a or an indirect heating type heater shown in FIG. 1 as a heating means for making the cooling rate of the steel plate M lower than the cooling rate of the cooling zone 3. As described above, by using the induction heater 5a or the indirect heating type heater as the heating means, the steel plate M can be gradually cooled or kept warm while the oxidation of the surface of the steel plate M is suppressed. Therefore, it can suppress that non-plating generate | occur | produces when manufacturing a plated steel plate.

  As an upper limit of the average cooling rate of the steel sheet M in the slow cooling and keeping zone 5, 10 ° C./s is preferable and 3 ° C./s is more preferable. By setting the average cooling rate to the upper limit or less, bainite can be partially formed in the steel sheet M. A carbon-enriched region is formed around the periphery, and this region is cooled and heated in the recooling zone 6 and the reheating zone 4, whereby coarse residual austenite can be formed. With this coarse retained austenite, the deep drawability and ductility of the steel sheet M after annealing can be enhanced. On the other hand, the lower limit of the average cooling rate is not particularly limited, and may be 0 ° C./s, that is, heat retention (maintaining the temperature of the steel plate M).

  The lower limit of the time (treatment time) for the steel sheet M to pass through the gradual cooling and warming zone 5 is preferably 10 seconds, and more preferably 50 seconds. On the other hand, the upper limit of the processing time is preferably 300 seconds, and more preferably 60 seconds. If the treatment time is less than the lower limit, the formation of the bainite becomes insufficient, and the deep drawability and ductility of the steel sheet M after annealing may be reduced. Conversely, if the treatment time exceeds the upper limit, the retained austenite that is promoted to form in the recooling zone 6 and the reheating zone 4 becomes too coarse due to the formation of bainite, and the hole expansion rate and tensile strength may decrease. There is.

  The lower limit of the cooling temperature of the steel sheet M in the slow cooling holding zone 5 (the end temperature of slow cooling or warming) is preferably 300 ° C and more preferably 340 ° C. On the other hand, the upper limit of the cooling temperature is preferably 500 ° C and more preferably 460 ° C. If the cooling temperature is less than the lower limit, the carbon concentration region in the steel sheet M becomes small, and coarse retained austenite cannot be secured, so that the deep drawability of the steel sheet M after annealing may be reduced. On the other hand, when the cooling temperature exceeds the upper limit, the carbon concentration region in the steel sheet M becomes large, and the retained austenite becomes too coarse, so that the hole expansion rate may be lowered.

  The lower limit of the length of the slow cooling and warming zone 5 in the conveyance direction is preferably three times the length of the cooling zone 3 in the conveyance direction, and more preferably 3.5 times. On the other hand, the upper limit of the length of the slow cooling and warming zone 5 in the transport direction is preferably 8 times the length of the cooling zone 3 in the transport direction, and more preferably 7 times. There exists a possibility that the ductility of the steel plate M after annealing may fall that the length of the conveyance direction of the slow cooling heat retention zone 5 is less than the said minimum. Conversely, if the length of the slow cooling and warming zone 5 in the transport direction exceeds the upper limit, the tensile strength of the steel sheet M after annealing may be reduced. That is, the amount of bainite formed in the slow cooling and warming zone 5 is controlled to an appropriate amount by setting the length in the transport direction of the slow cooling and warming zone 5 within the above range, so that the balance between strength and ductility is excellent. A steel plate M is obtained.

  Moreover, as a minimum of the length of the slow cooling warm zone 5 in the conveyance direction, 0.7 times the length of the slow cooling zone 2 in the conveyance direction is preferable, and 0.8 times is more preferable. On the other hand, the upper limit of the length of the slow cooling and warming zone 5 in the transport direction is preferably twice the length of the slow cooling zone 2 in the transport direction, and more preferably 1.9 times. There exists a possibility that the ductility of the steel plate M after annealing may fall that the length of the conveyance direction of the slow cooling heat retention zone 5 is less than the said minimum. Conversely, if the length of the slow cooling and warming zone 5 in the transport direction exceeds the upper limit, the tensile strength of the steel sheet M after annealing may be reduced. That is, the amount of bainite formed in the slow cooling and warming zone 5 is controlled to an appropriate amount by setting the length in the transport direction of the slow cooling and warming zone 5 within the above range, so that the balance between strength and ductility is excellent. A steel plate M is obtained.

  Furthermore, as a minimum of the length of the conveyance direction of the slow cooling heat retention zone 5, 30 m is preferable and 50 m is more preferable. Thus, by making the length of the slow cooling and warming zone 5 in the transport direction equal to or more than the above lower limit, the transport speed of the steel sheet M can be improved while securing the amount of bainite formed in the slow cooling and warming zone 5. Efficiency can be increased.

<Recooling zone>
In the recooling zone 6, the steel sheet M that has passed through the slow cooling and keeping zone 5 is cooled.

  The recooling zone 6 preferably includes a gas jet cooling device 6a as a cooling means for the steel plate M as shown in FIG. The gas jet cooling device 6a can perform cooling using a mixed gas of nitrogen and hydrogen. For this reason, by using the gas jet cooling device 6a as the cooling means, cooling can be performed while suppressing oxidation of the surface of the steel sheet M. Therefore, it can suppress that non-plating generate | occur | produces when manufacturing a plated steel plate.

  The average cooling rate of the steel sheet M in the recooling zone 6 is preferably 30 ° C./s or higher. When the average cooling rate is less than the above lower limit, the carbon enrichment region formed in the slow cooling heat insulation zone 5 becomes unnecessarily large during cooling, and the retained austenite becomes too coarse, resulting in a decrease in the hole expansion rate. There is a risk. On the other hand, the upper limit of the average cooling rate is not particularly limited, but is less than 200 ° C./s from the viewpoint of equipment and running cost.

  As a minimum of the cooling temperature (cooling end temperature) of the steel plate M in the recooling zone 6, 100 degreeC is preferable and 140 degreeC is more preferable. On the other hand, the upper limit of the cooling temperature is preferably 300 ° C, and more preferably 260 ° C. The cooling temperature in the recooling zone 6 mainly affects the amount of retained austenite remaining without transformation into martensite. There exists a possibility that the amount of retained austenite may become inadequate that the said cooling temperature is less than the said minimum. If the amount of retained austenite is insufficient, the tensile strength (TS) increases, but the total elongation (EL) tends to decrease greatly, and the balance (TS × EL) tends to deteriorate. On the contrary, when the cooling temperature exceeds the upper limit, the amount of retained austenite may be excessive. If the amount of retained austenite is excessive, the hole expansion rate tends to decrease.

<Reheating zone>
In the reheating zone 4, the steel sheet M that has passed through the recooling zone 6 is heated.

  As the heating means of the reheating zone 4, the same means as the heating zone 1 can be used.

  Although the average heating rate of the steel plate M in the reheating zone 4 is not particularly limited, it can be, for example, 1 ° C./s or more and 20 ° C./s or less. When the average heating rate is less than the lower limit, it takes time to reach a desired temperature, and thus production efficiency may be reduced. Conversely, if the average heating rate exceeds the upper limit, the equipment for heating and running costs may increase.

  The lower limit of the heating temperature (heating end temperature) of the steel sheet M in the reheating zone 4 is preferably 300 ° C and more preferably 340 ° C. On the other hand, the upper limit of the heating temperature is preferably 500 ° C and more preferably 460 ° C. When the heating temperature is less than the lower limit, carbon does not diffuse sufficiently and the amount of retained austenite becomes insufficient. On the other hand, when the heating temperature exceeds the upper limit, carbon tends to precipitate as cementite, and the amount of retained austenite becomes insufficient. For this reason, in any case, the balance (TS × EL) between the tensile strength and the total elongation of the steel sheet M after annealing may be deteriorated, and the hole expansion rate may be reduced.

  The steel sheet M that has passed through the reheating zone 4 is cooled to 200 ° C. or less at a cooling rate of 2 ° C./s or more and 20 ° C./s or less after, for example, an overaging treatment.

<Advantages>
Since the said steel plate continuous annealing equipment is equipped with the heating zone 1, the slow cooling zone 2, the cooling zone 3, and the reheating zone 4 in this order, it can manufacture many types of steel plates from a soft steel plate to a high strength steel plate. Moreover, the said steel plate continuous annealing equipment equips the cooling zone 3 and the reheating zone 4 with the slow cooling heat retention zone 5 and the recooling zone 6 in this order. By performing this heat treatment, a steel structure in which bainite, martensite, retained austenite, and unstable austenite are mixed can be obtained. The steel sheet continuous annealing equipment can adjust the balance of tensile strength, ductility, yield ratio, hole expandability, deep drawability, and the like at a high level by controlling the mixing ratio of these. Furthermore, in the said steel plate continuous annealing equipment, since it is not necessary to perform rapid cooling by water cooling, a plated steel plate can also be manufactured. Therefore, by using the steel plate continuous annealing equipment, it is possible to produce a wide variety of high-strength steel sheets and plated steel sheets according to various requirements with one equipment.

[Method of manufacturing annealed steel sheet]
The manufacturing method of the annealed steel plate concerning one embodiment of the present invention is provided with a heating process, a slow cooling process, a cooling process, a slow cooling heat retention process, a recooling process, and a reheating process. The manufacturing method of the said annealing steel plate can be performed using the steel plate continuous annealing equipment of this invention shown in FIG.

<Heating process>
In the heating step, the steel plate M is heated. This heating process can be performed in the heating zone 1 of the steel plate continuous annealing facility.

  The atmosphere of the heating zone 1 contains hydrogen and water vapor. Components other than hydrogen and water vapor in the atmosphere are preferably inert gases, and cheap nitrogen is more preferable.

  Further, the dew point of the atmosphere is controlled in order to promote the decarburization of the steel plate M and to control the properties of the steel plate surface. The lower limit of the dew point is −15 ° C., and −10 ° C. is more preferable. On the other hand, the upper limit of the dew point is 15 ° C, more preferably 10 ° C. If the dew point is less than the lower limit, the decarburization promoting effect of the steel plate M and the controllability of the properties of the steel plate M surface may be insufficient. On the contrary, when the dew point exceeds the upper limit, the iron foil easily adheres to the steel plate M or the equipment surface and the wall surface in the heating zone 1, and the occurrence of quality defects of the steel plate M may not be sufficiently suppressed. There is.

Further, when the temperature of the steel sheet surface in the heating zone 1 is T (K), the dew point is DP (K), and the gas constant is R (= 8.3144598 JK ⁻¹ mol ⁻¹ ), the hydrogen concentration H ( (Volume%) is made to satisfy the following formula (1).

  If the hydrogen concentration H in the atmosphere does not satisfy the above formula (1), the iron foil may easily adhere to the steel plate M or the equipment surface and the wall surface in the heating zone 1 in the heating zone 1. The present inventors speculate as follows about the mechanism by which iron foil is generated in the heating zone 1. First, when the steel plate M is placed in an atmosphere with a low hydrogen concentration in a state where the dew point is relatively high, the steel plate M is oxidized and the oxide is peeled off. When this oxide is reduced in the atmosphere of the heating zone 1, it becomes iron foil and adheres to the steel plate M and the equipment surface and wall surface in the heating zone 1. From the above reasoning, the present inventors predicted that the iron foil is reduced in the heating zone 1 by making the heating zone 1 an atmosphere in which the steel sheet M is hardly oxidized. And from the thermodynamic equilibrium conditions and the laboratory experiment results, the hydrogen concentration at which the iron foil decreases was derived, and the above formula (1) was obtained.

  Although the manufacturing method of the said annealed steel plate can be used for manufacture of various high-strength steel plates and plated steel plates, since it can suppress that an iron foil adheres to a steel plate as mentioned above, it is suitable for manufacture of a plated steel plate especially. is there. That is, the manufacturing method of the said annealing steel plate can suppress effectively plating peeling and non-plating by adhesion to the steel plate of iron foil. Moreover, since the manufacturing method of the said annealed steel plate can also suppress adhesion to the apparatus surface and wall surface in the heating heating zone 1 of iron foil, the cleaning time in the heating zone 1 can be reduced. Therefore, the manufacturing efficiency of the annealed steel sheet can be increased by the method for manufacturing the annealed steel sheet.

  In addition, the average heating rate and heating temperature in the said heating process can be as having mentioned with the heating zone 1 of the said steel plate continuous annealing equipment.

<Slow cooling process>
In the slow cooling step, the steel plate M after the heating step is gradually cooled. This slow cooling process can be performed in the soaking zone 7 and the slow cooling zone 2 of the steel plate continuous annealing equipment. That is, in the slow cooling step, the temperature of the steel sheet M is kept high for a certain time, and then the temperature is lowered.

  The atmosphere of the soaking zone 7 and the slow cooling zone 2 may be the same as the atmosphere of the heating zone 1, but may be other atmospheres such as an inert gas atmosphere such as nitrogen gas. Among these, since the soaking zone 7 is high temperature, it is preferable to make it the same as the atmosphere of the heating zone 1. By making the soaking zone 7 the same as the atmosphere of the heating zone 1, it is possible to more reliably prevent the iron foil from adhering to the steel plate or the equipment surface and the wall surface in the heating zone 1.

  In addition, the various conditions in the said slow cooling process can be as having described with the heat equalizing body 7 and the slow cooling zone 2 of the said steel plate continuous annealing equipment.

<Cooling process>
In the cooling step, the steel sheet M after the slow cooling step is cooled. This cooling process can be performed in the cooling zone 3 of the steel plate continuous annealing facility. The cooling rate and cooling temperature in the cooling step can be as described in the cooling zone 3 of the steel plate continuous annealing equipment.

<Slow cooling heat retention process>
In the slow cooling and keeping step, the steel sheet M after the cooling step is slowly kept warm. This slow cooling and warming step can be performed in the slow cooling and warming zone 5 of the steel sheet continuous annealing equipment. The average cooling rate, the treatment time, and the cooling temperature in the slow cooling and heating step can be as described in the slow cooling and warming zone 5 of the steel sheet continuous annealing equipment.

<Recooling zone process>
In the recooling step, the steel sheet M after the slow cooling and keeping step is recooled. This recooling step can be performed in the recooling zone 6 of the steel plate continuous annealing facility. The average cooling rate and cooling temperature in the recooling step can be as described in the recooling zone 6 of the steel plate continuous annealing facility.

<Reheating process>
In the reheating step, the steel sheet M after the recooling step is reheated. This reheating process can be performed in the reheating zone 4 of the steel plate continuous annealing facility. The average heating rate, the heating temperature, and the cooling after the heating in the reheating step can be performed as described in the reheating zone 4 of the steel plate continuous annealing facility.

<Advantages>
The manufacturing method of the said annealed steel plate can manufacture many types of high-strength steel plates and plated steel plates according to various individual requirements with one facility by using the steel plate continuous annealing facility of the present invention. In the heating process, the dew point is set within the above range and the hydrogen concentration is heated so as to satisfy the following formula (1). Therefore, the iron foil adheres to the steel plate M and the equipment surface and the wall surface in the heating zone 1 in the heating zone. It is difficult to suppress the occurrence of poor quality of the steel sheet M.

[Other Embodiments]
The present invention is not limited to the above embodiment.

  The steel plate continuous annealing equipment may include other processing bands in addition to the processing bands. For example, the said steel plate continuous annealing equipment may have a heat retention zone which maintains the temperature of the steel plate which passed the said cooling zone between the recooling zone and the reheating zone. As time for maintaining the temperature of a steel plate in the said heat retention zone, it can be 1 second or more and 600 seconds or less, for example.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples.

[Steel sheet continuous annealing equipment]
In order to investigate the effect of the steel sheet continuous annealing equipment of the present invention, the following evaluation was performed.

<Preparation of original plate>
A steel plate (original plate) before annealing was prepared by the following procedure. First, a cast material having the component composition shown in Table 1 (the balance is iron and inevitable impurities) was manufactured by vacuum melting, and then this cast material was hot forged to an average thickness of 30 mm. Next, this cast material was hot-rolled. In this hot rolling, the cast material was heated to 1200 ° C., and then the average thickness was 2.5 mm by multi-stage rolling. At this time, the end temperature of the hot rolling was 880 ° C. Thereafter, the obtained cast material was cooled to 600 ° C. at a cooling rate of 30 ° C./s, charged in a furnace heated to 600 ° C. for 30 minutes, and then cooled in a furnace to obtain a hot-rolled steel sheet. Further, the hot-rolled steel sheet was pickled to remove the surface scale, and then cold-rolled to obtain a cold-rolled steel sheet having an average thickness of 1.4 mm, which was used as an original sheet.

<Annealing treatment>
This original plate was heat-treated using a steel plate continuous annealing facility having the structure shown in FIG. 1-No. No. 4 steel sheet after annealing was obtained. Table 2 shows the conditions in each treatment zone. Moreover, in each Example, Table 3 shows the ratio of the length of the slow cooling and warming zone in the transport direction when the length of the cooling zone in the transport direction and the length of the slow cooling zone in the transport direction are used as references. . In addition, the steel plate after the annealing which passed the reheating zone was cooled after performing the overaging process for 400 second at 400 degreeC.

  In Table 2, the start temperature of each treatment zone is not specified, but since the steel sheet is processed while continuously passing through the treatment zone, the start temperature of each treatment zone is equal to the start temperature of the immediately preceding treatment zone. .

<Measurement>
No. 1-No. Yield strength YS, tensile strength TS, uniform elongation uEL, total elongation EL, and hole expansion ratio λ were measured for steel plate No. 4. The results are shown in Table 3.

  Yield strength YS, tensile strength TS, uniform elongation uEL, and total elongation EL were measured by a tensile test. Specifically, no. 1-No. The No. 5 test piece prescribed | regulated by JIS-Z2241 (2011) was extract | collected from the steel plate of 4 so that the direction perpendicular | vertical with respect to a rolling direction might become a longitudinal direction. Using this test piece, a tensile test was performed with a tensile tester under the conditions of JIS-Z2241 (2011) to determine yield strength YS, tensile strength TS, uniform elongation uEL, and total elongation EL. TS × EL in Table 3 was calculated as the product of the tensile strength TS and the total elongation EL.

The hole expansion ratio λ was determined according to the Japan Iron and Steel Federation standard JFS-T1001. Specifically, a punched hole having a diameter d0 (d0 = 10 mm) is formed in the test piece, a punch having a tip angle of 60 ° is pushed into the punched hole, and the punched hole when the generated crack penetrates the plate thickness of the test piece. The diameter d of was measured and calculated from the following formula (2).
λ (%) = {(d−d0) / d0} × 100 (2)

  From the results in Table 3, for example, No. 1-No. TS of 4 is 780 MPa or more, and it can be judged that the strength is excellent. No. 1-No. The EL of 4 is 12% or more, and it can be determined that the ductility is excellent. Furthermore, no. 1-No. Λ of 4 is 50% or more, and it can be determined that the workability such as press formability is excellent. Thus, it can be seen that the steel sheet continuous annealing equipment of the present invention can adjust the balance of the cord characteristics at a high level.

  In more detail, no. 2 and no. No. 3 steel is No. 3. Compared to steel No. 1, it has excellent EL and It is superior to TS in comparison with steel No. 4. That is, it can be seen that a steel material having a further excellent balance between strength and ductility can be obtained by setting the length in the transport direction of the slow cooling and warming zone to 3 to 8 times the length in the transport direction of the cooling zone. .

[Method of manufacturing annealed steel sheet]
In order to investigate the effect of the manufacturing method of the annealed steel sheet of the present invention, the following evaluation was performed by an experiment using a test apparatus.

<Preparation of original plate>
The steel plate (original plate) before annealing was prepared in the same procedure as the preparation of the original plate described in the evaluation of the steel plate continuous annealing equipment. In addition, the composition of the prepared steel plate is shown in Table 4.

<Heating process>
Under the conditions shown in Table 5, this original sheet was subjected to a heating step using the steel sheet continuous annealing equipment of the present invention. 5-No. A steel plate of 7 was obtained.

  First, after adjusting the time until the heating zone and the soaking atmosphere were stabilized under the conditions shown in Table 5, the heating zone was passed while heating the steel plate, and when the temperature of the steel plate surface reached 950 ° C, Passing through the soaking zone while keeping the temperature constant for 1 hour. Then, it cooled in nitrogen atmosphere in order to maintain the surface property of a steel plate.

<Measurement of oxide thickness>
About the steel plate obtained in this way, the cross section cut | disconnected in the thickness direction was expanded 5000 times with SEM (Scanning Electron Microscope, scanning electron microscope), and the oxide film on the surface was observed. As a result, no. 5 and no. No oxide film was observed on the steel plate No. 7, whereas no. In the steel plate No. 6, an oxide film having an average film thickness of 2.1 μm was observed. In addition, the average film thickness of the oxide film was calculated by measuring the thickness at twelve consecutive locations along the cross-section in the SEM image and averaging 10 points excluding the maximum value and the minimum value.

In Table 5, “Expression (1) right side” is a value obtained by calculating a calculation expression on the right side of the following expression (1).

  From the results of Table 5, the hydrogen concentration of No. 1 satisfying the above formula (1) is obtained. 5 and no. 7 shows that the oxide film thickness is 0 μm, and the steel sheet is not oxidized in the heating zone. On the other hand, no. In No. 6, since the hydrogen concentration does not satisfy the above formula (1), the steel plate is oxidized. From the above results, the steel sheet is oxidized by setting the dew point of the atmosphere at the time of heating to −15 ° C. or more and 15 ° C. or less and satisfying the above formula (1) with the hydrogen concentration H (volume%) of the atmosphere. It can be seen that this can be suppressed.

  As described above, the steel sheet continuous annealing equipment of the present invention can produce a wide variety of high-strength steel sheets and plated steel sheets according to various requirements with one equipment.

DESCRIPTION OF SYMBOLS 1 Heating zone 2 Slow cooling zone 2a Gas jet cooling device 3 Cooling zone 3a Gas jet cooling device 4 Reheating zone 5 Slow cooling insulation zone 5a Induction heater 6 Recooling zone 6a Gas jet cooling device 7 Soaking zone M Steel plate

Claims (7)

A heating zone for heating the steel plate, a slow cooling zone for gradually cooling the steel plate that has passed through the heating zone, a cooling zone for cooling the steel plate that has passed through the slow cooling zone, and a steel plate that has passed through the cooling zone are heated again. It is a continuous annealing facility for steel sheets having a reheating zone,
Between the cooling zone and the reheating zone,
A slow cooling and warming zone for slow cooling or warming the steel sheet that has passed through the cooling zone,
A steel plate continuous annealing facility comprising: a recooling zone that cools the steel plate that has passed through the slow cooling and warming zone again.   The steel sheet continuous annealing facility according to claim 1, wherein the slow cooling zone, the cooling zone, and the recooling zone include a gas jet cooling device as a cooling means for the steel plate.   The steel plate continuous annealing equipment according to claim 1 or 2, wherein the slow cooling and warming zone includes an induction heater or an indirect heating type heater as a heating means for making the cooling rate of the steel plate weaker than the cooling rate of the cooling zone. .   The steel plate continuous annealing facility according to claim 1, 2 or 3, wherein the length of the slow cooling and warming zone in the transport direction is not less than 3 times and not more than 8 times the length of the cooling zone in the transport direction.   The steel plate according to any one of claims 1 to 4, wherein a length of the slow cooling and warming zone in the transport direction is 0.7 to 2 times a length of the slow cooling zone in the transport direction. Continuous annealing equipment.   The steel sheet continuous annealing facility according to claim 4 or 5, wherein a length in the transport direction of the slow cooling and warming zone is 30 m or more. Heating the steel sheet;
A step of gradually cooling the steel sheet after the heating step;
Cooling the steel sheet after the slow cooling step;
A step of gradually cooling and keeping the steel sheet after the cooling step;
A step of recooling the steel sheet after the annealing step,
A step of reheating the steel sheet after the recooling step,
Using the steel sheet continuous annealing equipment according to claim 1,
The heating zone atmosphere in the heating step includes hydrogen and water vapor,
When the temperature of the steel sheet surface in the heating step is T (K), the dew point in the heating zone is DP (K), and the gas constant is R (= 8.3144598 JK ⁻¹ mol ⁻¹ ), the heating step The manufacturing method of the annealed steel plate which makes the dew point of the atmosphere at the time of heating -15 degreeC or more and 15 degrees C or less, and makes the hydrogen concentration H (volume%) of the said atmosphere satisfy | fill following formula (1).

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