JP6693498B2 - Equipment and method for manufacturing thick steel plate - Google Patents

Description

  The present invention relates to a thick steel plate manufacturing facility and manufacturing method.

  In recent years, a large-scale earthquake has occurred in Japan, and from the viewpoint of safety, especially in high-rise buildings, the strength of steel plates has been increased to prevent collapse of the building even if a large earthquake occurs. In addition to the tensile strength of 490 MPa), there is an increasing need for a low yield ratio steel having a yield ratio, which is the ratio of the yield strength to the tensile strength, of 80% or less. As a method for manufacturing a steel sheet having such a high strength and a low yield ratio, a method disclosed in Patent Document 1, for example, is conventionally known. In order to achieve a low yield ratio while maintaining high strength, the steel sheet structure has a suitable proportion of a soft ferrite phase (α) and a hard bainite (β) or martensite (M) according to the strength. Is widely used.

  Here, in the method for producing a low yield ratio low carbon low alloy high strength steel shown in Patent Document 1, a billet having a predetermined component is hot-rolled at a cumulative rolling reduction of 25% or more at 950 ° C. or less, After that, a so-called off-line heat treatment is performed using a heating furnace and a cooling device located in a place different from the hot rolling line. In this off-line heat treatment, the steel slab is heated to an appropriate temperature between the Ac1 transformation point and the Ac3 transformation point, and the transformed austenite phase is sufficiently air-cooled to obtain a martensite or a low temperature transformation product or a mixed structure of both. It is cooled at the above cooling rate and then tempered at an Ac1 transformation point temperature or lower. Steel sheets that have been heat treated off-line are commonly referred to as tempered steels.

On the other hand, as a method for obtaining a steel sheet having a low yield ratio without performing an off-line heat treatment, for example, those shown in Patent Documents 2 and 3 are known. The methods shown in Patent Documents 2 and 3 have a step of directly cooling a high-temperature steel sheet immediately after hot rolling, which is called an online heat treatment.
In the method for producing a high-toughness high-strength steel disclosed in Patent Document 2, a steel having a predetermined component is heated to 1000 ° C to 1300 ° C and hot-rolled to a cross-section rate of 80% or more in a temperature range of at least 980 ° C and Ar3. Immediately after that, the steel sheet is air-cooled or cooled to an Ar3 transformation temperature or lower at which ferrite is formed to generate ferrite first, and then the steel sheet is rapidly cooled to form a ferrite-martensite two-phase layered structure. It is an eggplant.

Further, in the method for producing a high tensile strength steel having a low yield ratio shown in Patent Document 3, after hot rolling steel having a predetermined composition range, the austenite fraction in the center part of the plate thickness is kept at 5 ° C / s or more until it reaches 90% or less. After cooling at a cooling rate, the temperature is raised from the Ac1 transformation point + 20 ° C to the Ac3 transformation point -20 ° C, and after heating and holding, the cooling is performed at a cooling rate of 5 to 30 ° C / s, and at 600 to 400 ° C. It will stop.
It should be noted that the steel sheet manufactured without the off-line heat treatment step as in the methods shown in Patent Documents 2 and 3 is generally called non-heat treated steel.

Further, as a control cooling device for thick steel plates that enables efficient mass production of low yield ratio thick steel plates, for example, one disclosed in Patent Document 4 is known.
The control cooling device for a thick steel sheet disclosed in Patent Document 4 is a slow cooling zone that gently cools a rolled thick steel sheet on a delivery side of a rolling mill and partially ferrite-transforms a surface layer of the thick steel sheet, and a slow cooling zone. With the rapid cooling zone for quenching the thick steel plate whose surface layer has undergone the ferrite transformation, and for controlling the transformation of the remaining austenite phase that has not undergone the ferrite transformation into pearlite, bainite, martensite, etc. An air cooling unit is provided between the two, and a thermometer for measuring the surface temperature of the steel sheet passing through the slow cooling zone is installed in this air cooling unit, and based on the temperature from this thermometer, the cooling stop temperature of the quenching zone is a predetermined value. Therefore, the cooling conditions such as the strip running speed and the cooling water amount are controlled.

JP-A-55-97425 JP-A-55-41927 JP-A-6-271934 JP, 2005-313223, A

By the way, generally, when a high temperature steel plate is water-cooled, film boiling with a small heat flux occurs in a high temperature region as shown in FIG. 12, and as the surface temperature of the steel plate decreases, a transition boiling state in which cooling is unstable is caused. After that, it becomes a nucleate boiling state.
Here, in the transition boiling region, the heat flux increases as the surface temperature of the steel sheet is lower, so if the steel sheet has a temperature deviation at the start of cooling, the temperature deviation increases as the cooling progresses. As long as the cooling is performed in the transition boiling region, the local temperature unevenness is integrated and expanded, and the material of the steel sheet after cooling varies.

  On the other hand, in the film boiling region and the nucleate boiling region, cooling is accelerated because the heat flux is high in the high temperature part because the heat flux is large in the high temperature part, whereas the cooling is delayed because the heat flux is small in the low temperature part, and as a result, both The temperature difference is reduced and the temperature unevenness is reduced. Therefore, when water-cooling a high temperature steel plate, uniform cooling is possible by avoiding the transition boiling region and cooling in the film boiling region or the nucleate boiling region. In general, film boiling tends to occur when the water density of the cooling water injected to the steel plate is low, and nucleate boiling tends to occur when the water density is high.Therefore, in order to achieve uniform cooling, avoid the transition boiling region and use low water density or high water density. Cool with water density.

Furthermore, the temperature at which the film boiling transitions to the transition boiling (hereinafter referred to as the transition boiling temperature) is affected by the scale generated on the surface of the steel sheet, and generally, the thicker the scale, the higher the transition boiling temperature. As a result of studies by the inventors of the present application, the relationship between the scale thickness and the temperature history of the steel sheet during cooling is as shown in FIG. 13, and the scale thickness is particularly susceptible to slow cooling with low water density. Considering the effect of scale, it is possible to more stably and uniformly cool the steel sheet by performing rapid cooling with high water density.
In addition, when a through-type cooling device widely used in recent years is used when performing cooling in online heat treatment, there is a deviation in the timing of entering the cooling device between the tip end and the tail end of the steel sheet. That is, when the transport speed of the steel sheet is V (m / s) and the length of the steel sheet is L (m), the tail end of the steel sheet is cooled by L / V (s) longer than the tip end. The cooling start temperature differs between the tip and tail of the steel sheet. FIG. 14 shows the temperature history of the tip and tail ends of the steel sheet during cooling. Since the cooling start temperature is different between the tip and the tail of the steel sheet, there is a problem that the ferrite fraction changes within the sheet and the yield ratio varies within the same steel sheet.

Here, in the method for manufacturing a low yield ratio low carbon low alloy high strength steel shown in Patent Document 1, the intermediate heat treatment temperature (intermediate temperature between the Ac1 transformation point and the Ac3 transformation point) in the off-line heat treatment should be appropriately selected. Although a steel sheet having a low yield ratio can be obtained with good reproducibility, a heat treatment step of heating and cooling a plurality of times is required after hot rolling, which increases energy cost and cannot prevent reduction in productivity of steel sheet.
Further, in the method of manufacturing a high toughness and high strength steel shown in Patent Document 2, since it is an online heat treatment and reheating is not performed after hot rolling, it is very advantageous from the viewpoint of energy cost. However, in the method shown in Patent Document 2, immediately after hot rolling, a steel sheet is air-cooled to a temperature not higher than the Ar3 transformation temperature at which ferrite is formed or cooled in accordance with it, but other materials are rolled during this waiting period. Cannot be achieved, which becomes an impediment to the productivity of the hot rolling line.

Further, in the method for producing a high-strength steel having a low yield ratio shown in Patent Document 3, although it is an online heat treatment, it is cooled at 5 ° C./s or more until the austenite fraction in the center part of the plate thickness becomes 90% or less. Since the temperature rising process is required after cooling at a speed, it is necessary to install a heating device in the hot rolling line. In Patent Document 3, there is no description of this heating device, and it is technically difficult to install such a heating device in the hot rolling line in the first place.
Further, as a problem of online heat treatment in the methods as disclosed in Patent Documents 2 and 3, as described above, the cooling start temperature is different between the front end and the tail end of the steel plate, so that the ferrite fraction changes within the plate. However, there is a problem that the yield ratio varies within the same steel sheet.

  Further, in the control cooling device for a thick steel sheet disclosed in Patent Document 4, the thick steel sheet rolled in the slow cooling zone is slowly cooled to partially generate ferrite in the surface layer of the thick steel sheet, and then rapidly cooled. The thick steel plate whose surface layer is ferrite-transformed in the slow cooling zone is rapidly cooled, and the remaining austenite phase that has not undergone ferrite transformation is transformation-controlled to pearlite, bainite, martensite, or the like. However, when the water amount density is low and the cooling is slow, as described above, it is easily affected by the scale thickness, the transition boiling temperature becomes high, the film boiling region becomes small, and the steel plate cannot be uniformly cooled. There is a problem that the materials vary. Further, in the case of the control cooling device for thick steel plate disclosed in Patent Document 4, the surface temperature of the steel plate passing through the slow cooling zone is measured by a thermometer installed in the air cooling section between the slow cooling zone and the rapid cooling zone, and the measurement is performed. Based on the results, it is possible to control the second-phase microstructure by controlling the cooling conditions such as strip speed and cooling water so that the cooling stop temperature in the quenching zone becomes a predetermined value. Cannot be controlled.

  Therefore, the present invention has been made to solve these conventional problems, and an object thereof is to reduce the energy cost and to reduce the productivity of the steel sheet, and It is an object of the present invention to provide a thick steel plate manufacturing facility and a manufacturing method capable of manufacturing a thick steel plate having high strength and a low yield ratio with little variation in temperature.

  In order to achieve the above object, the equipment for manufacturing a steel plate according to an aspect of the present invention includes a heating furnace that heats a steel plate at 100 ° C. or lower to an austenite temperature range, and a steel plate heated by the heating furnace. And a cooling device for cooling the thick steel plate, wherein the cooling device quenches the thick steel plate extracted from the heating furnace, and a transport direction of the first rapid cooling device. A second rapid cooling device that is installed on the downstream side and that rapidly cools the thick steel plate, each of the first rapid cooling device and the second rapid cooling device includes a plurality of pairs of upper and lower rapid cooling nozzles of the thick steel plate. While being arranged side by side along the transport direction, at least the upper side rapid cooling nozzles on the uppermost stream in the uppermost stream and the lowermost upstream side rapid cooling nozzles in each of the first rapid cooling device and the second rapid cooling device Draining device on the downstream side Arrangement, and the distance between the first rapid cooling device and the second quenching device is summarized in that to more than 5 m.

  Moreover, the manufacturing method of the thick steel plate which concerns on another aspect of this invention WHEREIN: The heating process which heats a thick steel plate of 100 degreeC or less to an austenite temperature range with a heating furnace, and the thick steel plate heated by this heating process with a cooling device. A method of manufacturing a thick steel sheet, comprising: a cooling step of cooling; the first cooling step of rapidly cooling the thick steel sheet extracted from the heating furnace by a first rapid cooling device of the cooling device. And the second quench of the cooling device installed at a position where the distance between the step and the steel plate rapidly cooled in the first quenching process is 5 m or more away from the first quenching device of the cooling device. An air cooling standby step of waiting for air cooling before cooling by a cooling device, and a second rapid cooling step of rapidly cooling the thick steel sheet standby in the air cooling standby step by the second rapid cooling device are provided. In each of the cooling step and the second rapid cooling step A plurality of pairs of upper and lower rapid cooling nozzles are arranged side by side along the transport direction of the thick steel plate to inject cooling water from the respective rapid cooling nozzles of the first and second rapid cooling devices into the thick steel plate. Rapid cooling is performed, and water is drained to at least the upstream side of the uppermost rapid cooling nozzle in the uppermost stream and the downstream side of the uppermost rapid cooling nozzle in the most downstream side of each of the first rapid cooling apparatus and the second rapid cooling apparatus. A device is arranged to prevent the cooling water from leaking to the outside of the cooling section by restraining the cooling water from the upper rapid cooling nozzle of each of the first rapid cooling device and the second rapid cooling device by the draining device. That is the summary.

  ADVANTAGE OF THE INVENTION According to the manufacturing equipment and manufacturing method of the thick steel plate which concern on this invention, it is high strength and low yield with few variations in a steel plate with the same yield ratio, without making energy cost low and reducing productivity of a steel plate. It is possible to provide a thick steel plate manufacturing facility and a manufacturing method capable of manufacturing a thick steel plate having a specific ratio.

It is a schematic block diagram of the heat processing equipment of the offline showing the manufacturing equipment of the thick steel plate which concerns on 1st Embodiment of this invention. It is the figure which demonstrated using the CCT diagram about the temperature history of the surface layer of the thick steel plate and the thickness direction center, and the microstructure change of the thick steel plate when it applies and cools the thick steel plate manufacturing equipment in this invention. After air cooling and air cooling, the initial cooling is set as rapid cooling (first rapid cooling), and then air cooling is performed where the ferrite fraction at the surface layer of the thick steel plate and the center in the plate thickness direction reaches a predetermined fraction (second cooling). (Cooling) to room temperature, and the second phase has a structure mainly composed of martensite. It is the figure which demonstrated using the CCT diagram about the temperature history of the surface layer of the thick steel plate and the thickness direction center, and the microstructure change of the thick steel plate when it applies and cools the thick steel plate manufacturing equipment in this invention. After air cooling and air cooling, the initial cooling is set as rapid cooling (first rapid cooling), and then air cooling is performed where the ferrite fraction at the surface layer of the thick steel plate and the center in the plate thickness direction reaches a predetermined fraction (second cooling). (Quench cooling), the quenching is stopped when the average temperature in the plate thickness direction of the thick steel plate reaches the bainite formation temperature, and the structure mainly composed of bainite is used as the second phase. It is the figure which demonstrated using the CCT diagram about the temperature history of the surface layer of the thick steel plate and the center of the plate thickness direction, and the microstructural change of the thick steel plate when it cooled by applying the manufacturing equipment of the thick steel plate which concerns on a reference example. It is a figure for demonstrating the method of air-cooling standby when the length of a thick steel plate is longer than the distance between a 1st rapid cooling device and a 2nd rapid cooling device. Drainage on the inlet side of the inlet side quenching nozzle in the first and second quenching devices, between the inlet side quenching nozzle and the outlet side quenching nozzle, and on the outlet side of the outlet side quenching nozzle. It shows the situation when the device is arranged and the case where the draining device is not arranged. (A) is a diagram showing the situation when the draining device is not arranged, (b) is the case when the draining device is arranged It is a figure which shows a situation. It is a figure similar to FIG.6 (b) at the time of changing the quenching nozzle in the 1st quenching apparatus and the 2nd quenching apparatus to the thing of a modification. It is a figure similar to FIG.6 (b) at the time of replacing a drainer with a modification. It is a figure similar to FIG.6 (b) at the time of changing the rapid cooling nozzle in a 1st rapid cooling device and a 2nd rapid cooling device to the thing of a modification, and changing the draining device to the thing of a modification. It is a schematic block diagram of the heat processing equipment of an offline showing the manufacturing equipment of the thick steel plate which concerns on 2nd Embodiment of this invention. It is a figure for demonstrating the calculation method of the air cooling standby time based on the measured value by a thermometer. It is a graph for explaining film boiling, transition boiling, and nucleate boiling. It is a graph which shows the relationship between the scale thickness at the time of cooling, and the temperature history of a steel plate. It is a graph which shows the temperature history of the front end and the tail end of a steel plate at the time of cooling.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments described below exemplify devices and methods for embodying the technical idea of the present invention, and the technical idea of the present invention is that the material, shape, structure, arrangement, etc. of components are It is not limited to the following embodiments. Further, the drawings are schematic. Therefore, it should be noted that the relationship between the thickness and the plane size, the ratio, and the like are different from the actual ones, and the drawings include portions in which the dimensional relationship and ratio are different from each other.

(First embodiment)
FIG. 1 shows a schematic configuration of an off-line heat treatment equipment representing a steel sheet manufacturing equipment according to a first embodiment of the present invention. The heat treatment equipment 1 is an off-line heat treatment equipment and has a temperature of 100 ° C. or less. The heating furnace 2 that heats the thick steel plate S up to the austenite temperature range, and the cooling device 3 that cools the thick steel plate S heated in the heating furnace 2. The heat treatment equipment 1 is used for heat treatment of a high-strength steel plate having a plate thickness of 12 mm to 100 mm and a tensile strength of 490 MPa or more with a low yield ratio (80% or less).

The heating furnace 2 is hot-rolled in advance to a predetermined thickness (for example, 15 mm), width (for example, 3000 mm) and length (for example, 15 m) in a hot rolling line (not shown) different from the heat treatment facility 1, After the temperature reaches room temperature, the thick steel plate S from which the scale on the steel plate surface has been removed by a scale removing mechanism (not shown) is charged. Then, in the heating furnace 2, the thick steel plate S is heated to the austenite temperature range (for example, about 910 ° C.).
The thick steel plate S extracted from the heating furnace 2 is cooled by the cooling device 3 while being conveyed by the plurality of table rolls 8 installed on the outlet side of the heating furnace 2.

Here, the cooling device 3 is installed on the first rapid cooling device 4 that rapidly cools the thick steel plate S extracted from the heating furnace 2, and on the downstream side in the transport direction of the first rapid cooling device 4, and the thick steel plate S is rapidly cooled. The second rapid cooling device 5 for cooling is provided.
The first quenching device 4 includes a plurality of pairs of upper quenching nozzles 6a and lower quenching nozzles 6b (five pairs in the present embodiment) that are paired up and down with respect to the transport line, and a thick steel plate. They are arranged side by side at a predetermined pitch along the transport direction of S. The cooling water 12 is jetted toward the thick steel plate S from each of the upper rapid cooling nozzle 6a and the lower rapid cooling nozzle 6b.

The second rapid cooling device 5 includes a plurality of pairs of upper and lower quenching nozzles 7a and 7b (four pairs in the present embodiment), which are paired vertically with respect to the transport line. The thick steel plates S are arranged side by side at a predetermined pitch along the conveying direction. The cooling water 12 is jetted toward the thick steel plate S from each of the upper rapid cooling nozzle 7a and the lower rapid cooling nozzle 7b.
Here, the heat treatment equipment 1 shown in FIG. 1 is an off-line type heat treatment equipment rather than an online type heat treatment equipment in order to reduce the variation of the yield ratio in the thick steel plate S, and is manufactured by using as little energy as possible. In order to reduce the cost, a thick steel sheet having a low yield ratio (yield ratio, which is a ratio of yield strength to tensile strength, of 80% or less) is manufactured in one heat treatment step (one heating).

  First, the reason why the offline heat treatment equipment is used instead of the online heat treatment equipment will be described. The biggest problem in the case of an online type heat treatment facility is that the mechanical characteristics vary within the same steel plate due to the temperature deviation between the tip and the tail end of the thick steel plate S before cooling. Therefore, in order to keep the cooling start temperature constant over the entire length of the thick steel plate S, an offline type heat treatment facility is used instead of the online type, the heating furnace 2 and the cooling device 3 are arranged in proximity, and the thick steel plate S is removed from the heating furnace 2. When the extraction is performed, cooling is performed by the cooling device 3 almost at the same time. As a result, it is possible to reduce the variation in the cooling start temperature between the tip and the tail of the thick steel plate S.

Here, the temperature decrease from the extraction of the tip and the tail of the thick steel plate S from the heating furnace 2 to the entry into the first rapid cooling device 4 of the cooling device 3 will be described.
In the heat treatment facility 1 shown in FIG. 1, in the heating furnace 2, the thick steel plate S is soaked and heated so as to have a constant temperature. Generally, since the plate temperature deviation of the thick steel plate S in the heating furnace 2 can be heated to about ± 5 to 10 ° C., and the thick steel plate S is heated to the furnace temperature of the heating furnace 2. The temperature inside the heating furnace 2 can be made substantially uniform with respect to the target temperature. Further, when the distance from the heating furnace 2 to the cooling device 3 is L ₀ (m), for example, the tip of the thick steel plate S is extracted from the heating furnace 2 and the thick steel plate S is conveyed at the transport speed V (m / s). It is assumed that the tip of the thick steel plate S has been conveyed and has entered the cooling device 3. In this case, since the temperature of the thick steel plate S starts to drop due to the outside air after the thick steel plate S exits the heating furnace 2, the tip of the thick steel plate S has a distance L ₀ (m) between the heating furnace 2 and the cooling device 3. It is cooled for the time L ₀ / V (s) of moving only. The distance L ₀ from the heating furnace 2 to the cooling device 3 is defined as the distance from the extraction port of the heating furnace 2 to the uppermost table roll 8 of the first rapid cooling device 4 as shown in FIG. ..

On the other hand, as for the tail end of the thick steel plate S, the tail end of the thick steel plate S is extracted from the heating furnace 2, the thick steel plate S is transported at a transport speed V (m / s), and the tail end of the thick steel plate S is a cooling device. Suppose you entered 3. In this case, since the temperature of the thick steel plate S starts to decrease due to the outside air after the thick steel plate S exits the heating furnace 2, the tail end of the thick steel plate S has a distance L ₀ (m between the heating furnace 2 and the cooling device 3). ) Moving time L ₀ / V (s) will be cooled. Here, if the transport speed V (m / s) of the thick steel plate S is constant, there is no temperature deviation at the leading end of the thick steel plate S in the heating furnace 2, and the thick steel plate S is allowed to cool for the same time and the cooling device 3 Therefore, there is no temperature deviation at the leading end of the thick steel plate S when the cooling device 3 enters. Therefore, there is little variation in the temperature of the leading end of the thick steel plate S for each plate material, and the cooling of the leading end of the thick steel plate S can be started at the same temperature. In order to start cooling at the same temperature for the tip end of the thick steel plate S, the tip end of the thick steel plate S leaves the heating furnace 2 until the tail end of the thick steel plate S enters the cooling device 3. It is preferable to pass the plate at the speed of.

  Further, since the cooling device 3 is composed of the first rapid cooling device 4 and the second rapid cooling device 5 and no slow cooling device is used, the following problems can be avoided. That is, when the water amount density is low and the cooling is slow, as described above, the scale thickness is easily affected, the transition boiling temperature becomes high, the film boiling region becomes small, and the steel sheet cannot be cooled uniformly. There is a problem that the materials vary. By using the rapid cooling device without using the slow cooling device, it is possible to cool in a nucleate boiling state, transition boiling does not occur, and it is possible to perform uniform cooling without being affected by scale.

Further, in the cooling device 3, the distance between the first rapid cooling device 4 and the second rapid cooling device 5 is set to 5 m or more. Here, the distance between the first rapid cooling device and the second rapid cooling device is, as shown in FIG. 1, the most downstream table roll 8 and the second rapid cooling device 5 in the first rapid cooling device 4. Is defined by the distance L _{1 from the} uppermost table roll 8 in FIG. The distance is set to 5 m or more because after the thick steel plate S is rapidly cooled by the first rapid cooling device 4, the thick steel plate S is allowed to stand by air cooling on the upstream side in the transport direction of the second rapid cooling device 5 for a predetermined time. In the thick steel plate S, ferrite is first generated in a predetermined fraction, and then the second rapid cooling device 5 is used to rapidly cool the remaining austenite phase to bainite phase or martensite phase. .. As a result, it is possible to disperse the soft ferrite phase and the hard phase bainite phase or martensite phase as a structure of the thick steel plate in an appropriate ratio according to the strength, and achieve a low yield ratio while maintaining the tensile strength. Because you can.

For example, in a thick steel plate S having a plate thickness of 30 mm, the surface layer of the thick steel plate S realized by a thick steel plate manufacturing facility in which the distance between the first rapid cooling device 4 and the second rapid cooling device 5 is 5 m or more. FIG. 2 using a CCT diagram shows the temperature history of the center of the plate thickness direction and the structural change of the thick steel plate.
After the thick steel plate S extracted from the heating furnace 2 is air-cooled (cooled), it is rapidly cooled by the first rapid cooling device 4 so that the average thickness of the thick steel plate S becomes 550 to 800 ° C. After waiting for air cooling, ferrite is first generated in a predetermined fraction. After that, rapid cooling is performed by the second rapid cooling device 5 to make the remaining austenite phase a bainite phase or martensite phase. As a result, the ferrite fraction of the first phase is controlled and the bainite phase or martensite phase of the second phase is formed in one heat treatment step. As a result, it is possible to disperse the soft ferrite phase and the hard phase bainite phase or martensite phase as a structure of the thick steel plate in an appropriate ratio according to the strength, and achieve a low yield ratio while maintaining the tensile strength. be able to. Further, since it is the off-line heat treatment equipment 1, it is possible to obtain a thick steel plate in which the ferrite fraction does not change within the plate and the yield ratio does not vary within the same steel plate.

Here, in FIG. 2, the thick steel plate S is cooled by the second rapid cooling device 5 until the plate thickness cross-section average temperature reaches room temperature. As shown in FIG. 3, the rapid cooling by the second rapid cooling device 5 may be stopped when the plate thickness cross-section average temperature of the thick steel plate S reaches 400 to 550 ° C. (bainite formation temperature). In this case, it is possible to create a structure in which ferrite is used as the first phase and bainite is used as the second phase.
On the other hand, FIG. 4 was explained using the CCT diagram for the temperature history of the surface layer of the thick steel sheet and the center of the thickness direction and the structural change of the thick steel sheet when the thick steel sheet manufacturing equipment according to the reference example was applied and cooled. It is a figure. The plate thickness of the thick steel plate is about 30 mm.

  In the thick steel plate manufacturing equipment according to the reference example, the thick steel plate S extracted from the heating furnace 2 is air-cooled (cooled) and then rapidly cooled to cool the thick steel plate to almost the water temperature. .. In this cooling method, after the rapid cooling, air cooling standby is not performed when the plate thickness cross-section average temperature of the thick steel plate S reaches 550 to 800 ° C., so that the surface layer of the thick steel plate S does not undergo ferrite transformation and martensite The site is undergoing a transformation. For this reason, in the thick steel plate manufacturing facility according to the reference example, it is not possible to form a multiphase structure in one heat treatment step.

  On the other hand, in the thick steel plate manufacturing facility according to the present embodiment, as described above, the cooling device 3 includes the first rapid cooling device 4 that rapidly cools the thick steel plate S extracted from the heating furnace 2. The first rapid cooling device 4 is provided downstream of the rapid cooling device 4 in the transport direction, and is provided with a second rapid cooling device 5 that rapidly cools the thick steel plate S, and the distance between the first rapid cooling device 4 and the second rapid cooling device 5 is set. Since it is 5 m or more, after the thick steel plate S is rapidly cooled by the first rapid cooling device 4, the thick steel plate S can be made to stand by air cooling on the upstream side in the transport direction of the second rapid cooling device 5 for a predetermined time. Ferrite is first generated in the thick steel plate S in a predetermined fraction, and then rapidly cooled by the second rapid cooling device 5 to make the remaining austenite phase a bainite phase or martensite phase. Therefore, the ferrite fraction of the first phase can be controlled and the bainite phase or martensite phase of the second phase can be formed in one heat treatment step.

The length of the thick steel plate S processed by a general off-line type heat treatment equipment is about 4 to 15 m. Therefore, if the distance between the first rapid cooling device 4 and the second rapid cooling device 5 is 5 m or more, the thick steel plate S with a plate length of 5 m or less is the first rapid cooling device 4 and the second rapid cooling device 5. Air cooling can be made to stand by. On the other hand, when the length of the thick steel plate S is longer than the distance L ₁ between the first rapid cooling device 4 and the second rapid cooling device 5, as shown in FIG. After passing through the first quenching device 4, as shown in FIG. 5 (b), the cooling water from the quenching nozzles 6a and 6b of the first quenching device 4 is stopped and the thick steel plate S is reversed to the upstream side. It is sufficient to convey the thick steel plate S to the downstream side and cool it by the second rapid cooling device 5, as shown in FIG.

If the distance between the first rapid cooling device 4 and the second rapid cooling device 5 is less than 5 m, in the case of a thick steel plate S having a long plate length, there arises a problem that the tail end of the thick steel plate S interferes with the heating furnace 2. Therefore, it is not possible to wait for air cooling in front of the second rapid cooling device 5.
Even for a long material, it is not necessary to control the reverse conveyance or the timing of stopping the injection of cooling water, and after cooling by the first rapid cooling device 4, the first rapid cooling device 4 and the second rapid cooling device 5 are connected. The distance between the first rapid cooling device 4 and the second rapid cooling device 5 is preferably 10 m or more, and more preferably 15 m or more, so that the air cooling standby can be performed. On the other hand, if the distance between the first rapid cooling device 4 and the second rapid cooling device 5 is too long, the equipment length becomes too long and there is a problem of equipment space. Therefore, it is preferably 25 m or less.

Further, in the cooling device 3, the water amount density of the cooling water in each of the first and second rapid cooling devices 4 and 5 is 1.0 m ³ / (min · m ² ) or more and 4.0 m ³ / (min · m ² ) or less. As a result, rapid cooling can be realized, the cooling rate can be controlled to 4 ° C./s or more at the center of the thick steel plate S in the plate thickness direction, and a uniform material in the thick steel plate S, in particular, a high strength low with a small yield ratio variation. A yield ratio tempered steel sheet can be manufactured. By setting the water amount density to 1.0 m ³ / (min · m ² ) or more, cooling can be performed in a nucleate boiling state, transition boiling does not occur, and cooling is performed uniformly without being affected by scale. You can When the water amount density is less than 1.0 m ³ / (min · m ² ), the cooling is slow, transitional boiling occurs and temperature unevenness occurs, and the material of the thick steel plate S after cooling, especially the variation of the yield ratio is large. Become. In order to cool the thick steel plate S more uniformly, the water amount density is preferably 1.2 m ³ / (min · m ² ) or more, more preferably the water amount density is 1.5 m ³ / (min · m ² ) or more. It is good to say On the other hand, if the water density is too large, the equipment cost will be enormous. Therefore, the upper limit of the water density is preferably set to 4.0 m ³ / (min · m ² ).

  Further, in the first rapid cooling device 4, as shown in FIG. 1, between the upper rapid cooling nozzles 6a adjacent to the conveyance direction in the first rapid cooling device 4 and upstream of the uppermost rapid cooling nozzle 6a in the uppermost stream. Side and the downstream side of the uppermost rapid cooling nozzle 6a located at the most downstream side, the draining device 9 is arranged. Similarly, in the second rapid cooling device 5 as well, it is located between the upper rapid cooling nozzles 7a adjacent to each other in the transport direction of the second rapid cooling device 5 and on the upstream side and the downstream side of the uppermost rapid cooling nozzle 7a located at the uppermost stream. A drainer 9 is arranged downstream of the upper quench nozzle 7a. Each of the draining device 9 in the first rapid cooling device 4 and the draining device 9 in the second rapid cooling device 5 is composed of a draining roll.

  As described above, the reason why the water draining device 9 is installed in each of the first rapid cooling device 4 and the second rapid cooling device 5 is as shown in FIG. The cooling water 12 jetted from the rapid cooling nozzles 6a and 7a leaks to the outside of the cooling section as leakage water 13, and the temperature of the thick steel plate S becomes lower than the target temperature due to the cooling water staying on the thick steel plate S. There is a problem. On the other hand, as shown in FIG. 6B, when the water draining device 9 is installed in each of the first rapid cooling device 4 and the second rapid cooling device 5, the first rapid cooling device 4 and the second rapid cooling device The cooling water 12 from the respective upper rapid cooling nozzles 6a and 7a of 5 can be restrained by the drainer 9 to prevent the leakage of the cooling water 12 to the outside of the cooling section. Thereby, stable cooling of the thick steel plate S can be realized. Particularly, when the water draining device 9 is installed in the second rapid cooling device 5, when the rapid cooling is stopped so that the plate thickness cross-section average temperature of the thick steel plate S becomes 400 to 550 ° C. in the second rapid cooling device 5, cooling is performed. The cooling water 12 can be prevented from leaking to the outside of the section, and the cooling stop temperature can be accurately controlled.

  Here, each draining device 9 is located above the transport line and has an elevating function, so that the thick steel plates S having various plate thicknesses can be restrained with a constant pressing force. In order to obtain good drainage property, it is preferable to make the surface shape of the thick steel plate S during cooling flat, and the pressing force of each drainer 9 is preferably 4 tons or more, more preferably 6 tons or more, further preferably 8 tons or more. Good. On the other hand, if the pressing force of each draining device 9 is too large, the draining device (draining roll) 9 may bend and a gap may occur between the thick steel plate S and the draining device 9 to deteriorate the draining property. The pressing force is preferably 20 tons or less.

The upper quenching nozzles 6a, 7a and the lower quenching nozzles 6b, 7b in each of the first quenching device 4 and the second quenching device 5 are spray-cooled with a collision shape of a circle, an ellipse or a rectangle. is there.
The upper quenching nozzles 6a, 7a in each of the first quenching device 4 and the second quenching device 5 are circular pipe nozzles provided in the cooling header 60a, as shown in FIGS. 7 and 9. However, a circular jet may be jetted from this circular nozzle. Further, the lower rapid cooling nozzles 6b and 7b in each of the first rapid cooling device 4 and the second rapid cooling device 5 are circular pipe nozzles provided in the cooling header 60b as shown in FIGS. 7 and 9. Alternatively, the circular pipe jet flow 14 may be ejected from this circular pipe nozzle.

Further, each draining device 9 may be configured not by a draining roll, but by a purge nozzle that jets the purge water 15 and restrains the cooling water, as shown in FIGS. 8 and 9. However, in order to perform more stable cooling without leaking the cooling water to the outside of the water cooling application section, it is preferable to use a water draining roll as the water draining device 9.
7 shows an example in which circular pipe nozzles are adopted as the upper quenching nozzles 6a, 7a and lower quenching nozzles 6b, 7b, and a draining roll is adopted as the draining device 9, and FIG. Shows an example in which a cooling spray is adopted as the upper rapid cooling nozzles 6a and 7a and lower rapid cooling nozzles 6b and 7b, and a purge nozzle is adopted as the water draining device 9. FIG. 9 shows the upper rapid cooling nozzles. There is shown an example in which circular pipe nozzles are adopted as the 6a and 7a and the lower rapid cooling nozzles 6b and 7b, and a purge nozzle is adopted as the drainer 9.

Next, in the heat treatment equipment 1, the cooling water is jetted so that the average thickness of the thick steel plate S at the end point of the first rapid cooling device 4 becomes a target temperature within the range of 550 ° C to 800 ° C. Control the number of quenching nozzles 6a, 6b (logarithm of quenching nozzles 6a, 6b), the water density of the cooling water in the first quenching device 4, and the transport speed of the thick steel plate S in the first quenching device 4. A control device 10 for controlling the operation is provided.
The control device 10 functions as a first rapid cooling control device, is connected to the upper rapid cooling nozzle 6a and the lower rapid cooling nozzle 6b in the first rapid cooling device 4, and is connected to the host computer 11. It is connected.

Then, the control device 10, in addition to the information such as the heating temperature and the plate thickness, from the host computer 11, the plate thickness cross-section average temperature of the thick steel plate S at the end point of the first rapid cooling device 4 is 550 ° C. to 800 ° C. Acquire information on the target temperature within the range of ° C. Then, the control device 10 controls the number of the rapid cooling nozzles 6a and 6b for injecting the cooling water such that the average thickness of the thick steel plate S at the end point of the first rapid cooling device 4 becomes the target temperature (quick cooling). The logarithm of the nozzles 6a and 6b), the water amount density of the cooling water in the first rapid cooling device 4, and the transport speed of the thick steel plate S in the first rapid cooling device 4 are calculated. As described above, the water quantity density of the cooling water in the first rapid cooling device 4 is set to 1.0 m ³ / (min · m ² ) or more and 4.0 m ³ / (min · m ² ) or less. Then, the controller 10 conveys the thick steel plate S at the calculated conveying speed in the first rapid cooling device 4 and calculates the calculated number of the rapid cooling nozzles 6a and 6b (logarithm of the rapid cooling nozzles 6a and 6b). ) And the water quantity density of the cooling water in the first rapid cooling device 4, the cooling water is jetted from the rapid cooling nozzles 6a and 6b toward the thick steel plate S. Thereby, the plate thickness cross-section average temperature of the thick steel plate S at the end point of the first rapid cooling device 4 is set to a target temperature within the range of 550 ° C to 800 ° C.

In the heat treatment equipment 1, the control device 10 also functions as a second rapid cooling control device, and the average thickness of the thick steel plate S at the end point of the second rapid cooling device 5 is from room temperature to 550 ° C. Number of rapid cooling nozzles 7a, 7b in the second rapid cooling device 5 that injects cooling water (logarithm of the rapid cooling nozzles 7a, 7b), cooling in the second rapid cooling device 5 so as to stop at the target temperature within the range The water amount density of water and the transport speed of the thick steel plate S in the second rapid cooling device 5 are controlled.
Specifically, the control device 10 is connected to the upper rapid cooling nozzle 7a and the lower rapid cooling nozzle 7b in the second rapid cooling device 5, and is also connected to the host computer 11.

Then, the control device 10, in addition to the information such as the heating temperature and the plate thickness from the host computer 11, the average thickness of the thick steel plate S at the end point of the second rapid cooling device 5 from room temperature to 550 ° C. Acquires information on the target temperature within the range. Then, the control device 10 controls the number of the rapid cooling nozzles 7a, 7b for injecting the cooling water so that the average thickness of the thick steel plate S at the end point of the second rapid cooling device 5 becomes the target temperature (rapid cooling). The logarithm of the nozzles 7a and 7b), the water density of the cooling water in the second rapid cooling device 5, and the transport speed of the thick steel plate S in the second rapid cooling device 5 are calculated. The water amount density of the cooling water in the second rapid cooling device 5 is, as described above, 1.0 m ³ / (min · m ² ) or more and 4.0 m ³ / (min · m ² ) or less. Then, the control device 10 conveys the thick steel plate S at the calculated conveyance speed in the second rapid cooling device 5 and calculates the calculated number of the rapid cooling nozzles 7a and 7b (logarithm of the rapid cooling nozzles 7a and 7b). ) And the water quantity density of the cooling water in the second rapid cooling device 5, the cooling water is jetted from the rapid cooling nozzles 7a, 7b toward the thick steel plate S. Thereby, the plate thickness cross-section average temperature of the thick steel plate S at the end point of the second rapid cooling device 5 is set to a target temperature within the range of room temperature to 550 ° C.

Further, in the heat treatment facility 1, the thick steel plate S heated in the heating furnace 2 is one in which the scale on the surface of the steel plate has been removed by a scale removing mechanism (not shown) in advance before heating in the heating furnace 2. The distance from 2 to the first rapid cooling device 4 is 4 m or less. Thereby, the thick steel plate S can be cooled more uniformly without being affected by the scale.
A scale produced during hot rolling is formed on the surface of the thick steel plate S before being charged into the heating furnace 2. This scale generally has a thickness of about 10 to 50 μm, but by removing the scale in advance with a scale removing mechanism, for example, a step such as shot blasting or pickling, the scale of the scale before being inserted into the heating furnace 2 is reduced. The thickness can be less than 1 μm. In addition, it is preferable to suppress the generation of scale in the furnace by heating the inside of the heating furnace 2 in a non-oxidizing atmosphere such as a nitrogen atmosphere, and it is preferable that the oxygen concentration in the furnace is less than 1%.

Before heating in the heating furnace 2, the scale on the surface of the steel sheet is removed by a scale removing mechanism in advance, and the inside of the heating furnace 2 is heated in a non-oxidizing atmosphere, so that the surface of the thick steel sheet S until extraction from the heating furnace 2 Although it is possible to prevent the generation of scale, after the heating furnace 2 is extracted, the thick steel plate S at high temperature is exposed to the atmospheric atmosphere and scale is also generated. Therefore, from the viewpoint of suppressing the scale, the time from the extraction of the heating furnace 2 to the infiltration into the cooling device 3 is at least 120 sec or less, preferably 100 sec or less. The transfer speed in a general off-line type heat treatment equipment is about 2 to 20 mpm, and from the viewpoint of preventing scale as much as possible, when the minimum transfer speed is 2 mpm, from the heating furnace 2 to the first quenching device 4. The distance is preferably 4 m or less, more preferably 3.3 m or less.
Further, if the distance from the heating furnace 2 to the first rapid cooling device 4 is too short, the cooling water scattered from the first rapid cooling device 4 enters the heating furnace 2 and the heating furnace 2 deteriorates. Therefore, the distance from the heating furnace 2 to the first rapid cooling device 4 is preferably 0.5 m or more.

Next, a method for manufacturing a thick steel plate using the heat treatment equipment 1 shown in FIG. 1 will be described.
First, a hot rolling line (not shown) different from the heat treatment facility 1 was hot rolled in advance to a predetermined thickness (for example, 15 mm), width (for example, 3000 mm) and length (for example, 15 m) to reach room temperature. After that, the thick steel plate S from which the scale has been removed by a scale removing mechanism (not shown) is charged into the heating furnace 2. Then, in the heating furnace 2, the thick steel plate S is heated to an austenite temperature range (for example, about 910 ° C.) (heating step).

Next, the thick steel plate S is extracted from the heating furnace 2, conveyed by the plurality of table rolls 8 installed on the outlet side of the heating furnace 2, and cooled by the cooling device 3 (cooling step).
In this cooling process, first, the thick steel plate S extracted from the heating furnace 2 is rapidly cooled by the first rapid cooling device 4 of the cooling device 3 arranged within 4 m from the heating furnace 2 (first rapid cooling process). ).
In this first rapid cooling step, cooling water is supplied from the rapid cooling nozzles 6a and 6b of the first rapid cooling device 4 in which a plurality of pairs of upper and lower rapid cooling nozzles 6a and 6b are arranged side by side along the transport direction of the thick steel plate S. It is injected into S to perform rapid cooling.

Here, in the first rapid cooling step, the water amount density of the cooling water in the first rapid cooling device 4 is set to 1.0 m ³ / (min · m ² ) or more and 4.0 m ³ / (min · m ² ) or less and the Cooling water is jetted from the cooling nozzles 6a and 6b to the thick steel plate S to perform rapid cooling.
Further, in the first rapid cooling step, the cooling water 12 from the upper rapid cooling nozzle 6a of the first rapid cooling device 4 is restrained by the drainer 9 to prevent the leakage of the cooling water to the outside of the cooling section.
Further, in the first rapid cooling step, the controller 10 sets the average thickness of the thick steel sheet S at the end point of the first rapid cooling device 4 to the target temperature within the range of 550 ° C to 800 ° C. Rapid cooling is performed by controlling the number of rapid cooling nozzles 6a and 6b for injecting cooling water, the water amount density of the cooling water in the first rapid cooling device 4, and the transport speed of the thick steel plate S in the first rapid cooling device 4. ..

Next, in the cooling process, the thick steel plate S rapidly cooled in the first quenching process is placed in a position where the distance between the thick steel plate S and the first quenching device 4 of the cooling device 3 is 5 m or more. Before being cooled by the second rapid cooling device 5, the air cooling is waited (air cooling waiting step).
Then, in the cooling step, the thick steel plate S that has been air-cooled in the air-cooling standby step is rapidly cooled by the second rapid cooling device 5 (second rapid cooling step).
In the second rapid cooling step, cooling water is supplied from the rapid cooling nozzles 7a and 7b of the second rapid cooling device 5 in which a plurality of pairs of upper and lower rapid cooling nozzles 7a and 7b are arranged side by side along the transport direction of the thick steel plate S. It is injected into S to perform rapid cooling.

Here, in the second rapid cooling step, the water amount density of the cooling water in the second rapid cooling device 5 is set to 1.0 m ³ / (min · m ² ) or more and 4.0 m ³ / (min · m ² ) or less to achieve the rapid cooling. Cooling water is jetted from the cooling nozzles 7a and 7b to the thick steel plate S to perform rapid cooling.
In the second rapid cooling step, the cooling water 12 from the upper rapid cooling nozzle 7a of the second rapid cooling device 5 is restrained by the drainer 9 to prevent the leakage of the cooling water to the outside of the cooling section.
Further, in the second rapid cooling step, the controller 10 controls the thick steel sheet S to stop at the target temperature in the range of room temperature to 550 ° C. at the plate thickness cross-section average temperature at the end point of the second rapid cooling device 5. The rapid cooling is performed by controlling the number of the rapid cooling nozzles 7a and 7b for injecting the cooling water, the water amount density of the cooling water in the second rapid cooling device 5, and the transport speed of the thick steel plate S in the second rapid cooling device 5. ..
Then, the thick steel plate S that has undergone the cooling step is subjected to a post step.

  As described above, according to the method for manufacturing the thick steel plate S according to the first embodiment, the thick steel plate S extracted from the heating furnace 2 has a plate thickness cross-section average temperature of 550 in the first rapid cooling step. After rapid cooling to ˜800 ° C., air cooling standby is performed in the air cooling standby process to first generate ferrite in a predetermined fraction. After that, rapid cooling is performed in the second rapid cooling step to stop the thick steel sheet S at a target temperature in the range of room temperature to 550 ° C. at the end point of the second rapid cooling device 5, where the average sheet thickness cross-section temperature is The remaining austenite phase is the bainite phase or martensite phase. As a result, the ferrite fraction of the first phase is controlled and the bainite phase or martensite phase of the second phase is formed in one heat treatment step. As a result, it is possible to disperse the soft ferrite phase and the hard phase bainite phase or martensite phase as a structure of the thick steel plate in an appropriate ratio according to the strength, and achieve a low yield ratio while maintaining the tensile strength. be able to. Since the thick steel sheet S having a low yield ratio can be obtained in one heat treatment step, the energy cost can be reduced by manufacturing the steel sheet S with the least possible energy.

(Second embodiment)
Next, the manufacturing equipment and the manufacturing method of the thick steel plate according to the second embodiment of the present invention will be described with reference to FIGS. 10 and 11.
FIG. 10 shows a schematic configuration of an off-line heat treatment equipment representing a steel sheet manufacturing equipment according to the second embodiment of the present invention. The heat treatment equipment 1 shown in FIG. 10 is the heat treatment equipment 1 shown in FIG. And the basic configuration is the same, but the control device 10 has a function of controlling the air cooling standby time by providing the thermometer 20, and the control device (first rapid cooling control device and second rapid cooling control device). 10 is equipped with a transport speed control function for controlling the transport speed of the thick steel plate S in the first rapid cooling device 4 and the transport speed of the thick steel plate S in the second rapid cooling device 5 to be the same. ing.

More specifically, in the heat treatment facility 1, between the first and second rapid cooling devices 4 and 5, the surface temperature of the thick steel plate S at the time of completion of cooling by the first rapid cooling device 4 is measured. A thermometer 20 is installed.
Further, the control device 10 functions as an air-cooling standby time control device, and in front of the second rapid cooling device 5 for allowing the structure of the thick steel plate S to have a desired ferrite fraction based on the measurement value by the thermometer 20. Transporting the thick steel plate S so that the thick steel plate S exiting the first rapid cooling device 4 is cooled by the second rapid cooling device 5 after the calculated air cooling standby time elapses. A control device 10 for controlling the speed is provided.

The control device 10 is connected to the table roll 8 and the thermometer 20, and is also connected to the host computer 11. Then, the control device 10 acquires the surface temperature of the target thick steel plate S after completion of cooling by the first rapid cooling device 4 calculated by the host computer 11, and cools it using the heat treatment equipment 1 shown in FIG. 10. The temperature history of the surface layer of the thick steel plate S and the center of the plate thickness direction and the information indicating the structural change of the thick steel plate are acquired, while the measurement value from the thermometer 20 (when the cooling by the first rapid cooling device 4 is completed) Surface temperature of the thick steel plate S). Then, the control device 10 corrects the surface temperature of the target thick steel plate S after the completion of cooling by the first rapid cooling device 4 calculated by the host computer 11 to the measurement value acquired from the thermometer 20. Next, the control device 10 controls the time t ₁ from the temperature measurement point (obtained from the thermometer 20) shown in FIG. 11 to the Ps line where the ferrite transformation starts and the thick steel plate S to be the target ferrite fraction. The holding time t ₂ is calculated, and the air cooling standby time (t ₁ + t ₂ ) is calculated. Here, when the retention time t ₂ is calculated, for example, the transformation rate is calculated by the Johnson-Mehl-Avrami equation to obtain the retention time t ₂ that is the target transformation rate. Then, the control device 10 controls the thickness so that the thick steel plate S exiting the first rapid cooling device 4 is cooled by the second rapid cooling device 5 after the elapse of the calculated air cooling standby time (t ₁ + t ₂ ). The conveyance speed of the steel plate S is controlled. Thereby, even when the measured value acquired from the thermometer 20 is out of the surface temperature of the target thick steel plate S after the cooling by the first rapid cooling device 4 calculated by the host computer 11, the air cooling standby time is set. By controlling, the ferrite fraction can be controlled with high accuracy, and a high-strength, low-yield-ratio tempered steel sheet with a small yield ratio variation can be manufactured.

Further, in the heat treatment equipment 1, the thick steel plate S passes through the inside of the first rapid cooling device 4 and the inside of the second rapid cooling device 5 at the same speed, so that the air cooling standby time in front of the second rapid cooling device 5 is increased. It is possible to make the front end portion and the tail end portion of the steel sheet S the same, and it is possible to manufacture a high-strength, low-yield-ratio heat-treated steel sheet with a small yield ratio variation in the longitudinal direction of the thick steel sheet S.
Here, when the air-cooling standby time at the tip of the thick steel plate S is t _Top and the air-cooling standby time at the tail end of the thick steel plate S is t _Bot , t _Top and t _Bot are respectively the following equation (1) and It is expressed by equation (2).
t _Top = t ₀ + L ÷ V ₁ (1)
t _Bot = t ₀ + L ÷ V ₂ (2)

Here, L is the plate length of the thick steel plate S, V ₁ is the plate passing speed in the first rapid cooling device 4, V ₂ is the plate passing speed in the second rapid cooling device 5, and t ₀ is the tail of the thick steel plate S. This is the time from the end of the cooling by the first rapid cooling device 4 to the start of the cooling by the second rapid cooling device 5 at the tip of the thick steel plate S.
When the strip running speed in the first rapid cooling device 4 and the strip running speed in the second rapid cooling device 5 are different (V ₁ ≠ V ₂ ), as can be seen from the equations (1) and (2), The air cooling standby time at the tip of the steel plate S and the air cooling standby time at the tail end of the thick steel plate S are different. On the other hand, when the strip passing speed in the first rapid cooling device 4 and the strip passing speed in the second rapid cooling device 5 are the same (V ₁ = V ₂ ), the same air cooling standby time is set over the entire length of the thick steel plate S. You can

Here, in the heat treatment equipment 1, the control device (the first rapid cooling control device and the second rapid cooling control device) 10 controls the transport speed of the thick steel plate S in the first rapid cooling device 4 and the second rapid cooling device 5. It also has a function as a transport speed control device for controlling the transport speed of the thick steel plate S therein to be the same.
In addition to the information such as the heating temperature and the plate thickness from the host computer 11, the control device 10 controls the plate thickness cross-section average temperature of the thick steel plate S of 550 ° C. to 800 ° C. at the end point of the first rapid cooling device 4. The target temperature information in the range is acquired, and the target temperature information in the range of room temperature to 550 ° C., which is the average thickness of the thick steel plate S at the end point of the second rapid cooling device 5, is obtained.

Then, the control device 10 sets the target temperature within the range of 550 ° C. to 800 ° C. for the plate thickness cross-section average temperature of the thick steel plate S at the end point of the first rapid cooling device 4, and controls the second rapid cooling of the thick steel plate S. On the premise that the plate thickness cross-section average temperature at the end point of the cooling device 5 is stopped at the target temperature within the range of room temperature to 550 ° C., the transport speed of the thick steel plate S in the first rapid cooling device 4 and The number of the rapid cooling nozzles 6a and 6b in the first rapid cooling device 4 that injects cooling water such that the conveyance speed of the thick steel plate S in the second rapid cooling device 5 is the same (the number of the rapid cooling nozzles 6a and 6b is Logarithm), the water quantity density of the cooling water in the first rapid cooling device 4, the number of the rapid cooling nozzles 7a, 7b in the second rapid cooling device 5 for injecting the cooling water (the logarithm of the rapid cooling nozzles 7a, 7b), the second rapid cooling device. Water quantity density of cooling water in cooling device 5 and first rapid cooling device 4 and the second to calculate the conveying speed of the steel plate S in the rapid cooling device 5. However, regarding the water amount density of the cooling water in each of the first rapid cooling device 4 and the second rapid cooling device 5, as described above, 1.0 m ³ / (min · m ² ) or more and 4.0 m ³ / (min・ M ² ) or less.

  Then, the control device 10 conveys the thick steel plate S in the first rapid cooling device 4 and the second rapid cooling device 5 at the calculated transport speed of the thick steel plate S at the same speed while calculating the calculated first rapid steel plate S. Number of rapid cooling nozzles 6a, 6b in the cooling device 4 (logarithm of the rapid cooling nozzles 6a, 6b), water density of cooling water in the first rapid cooling device 4, rapid cooling in the second rapid cooling device 5 for injecting cooling water The cooling water is jetted at the number of nozzles 7a and 7b (logarithm of the rapid cooling nozzles 7a and 7b) and the water density of the cooling water in the second rapid cooling device 5. Thereby, the strip passing speed in the first rapid cooling device 4 and the strip rapid speed in the second rapid cooling device 5 are made the same, the air cooling standby time is made the same over the entire length of the thick steel plate S, and the heating furnace 2 is extracted. The temperature history from the end to the end of cooling can be made uniform over the entire length of the thick steel plate S, and it is possible to manufacture a high-strength, low-yield-ratio tempered steel plate with a small variation in the yield ratio in the longitudinal direction of the thick steel plate S. ..

Next, a method of manufacturing a thick steel plate using the heat treatment equipment 1 shown in FIG. 10 will be described.
First, a hot rolling line (not shown) different from the heat treatment facility 1 was hot rolled in advance to a predetermined thickness (for example, 15 mm), width (for example, 3000 mm) and length (for example, 15 m) to reach room temperature. After that, the thick steel plate S from which the scale has been removed by a scale removing mechanism (not shown) is charged into the heating furnace 2. Then, in the heating furnace 2, the thick steel plate S is heated to an austenite temperature range (for example, about 910 ° C.) (heating step).
Next, the thick steel plate S is extracted from the heating furnace 2, conveyed by the plurality of table rolls 8 installed on the outlet side of the heating furnace 2, and cooled by the cooling device 3 (cooling step).

In this cooling process, first, the thick steel plate S extracted from the heating furnace 2 is rapidly cooled by the first rapid cooling device 4 of the cooling device 3 arranged within 4 m from the heating furnace 2 (first rapid cooling process). ).
In this first rapid cooling step, cooling water is supplied from the rapid cooling nozzles 6a and 6b of the first rapid cooling device 4 in which a plurality of pairs of upper and lower rapid cooling nozzles 6a and 6b are arranged side by side along the transport direction of the thick steel plate S. It is injected into S to perform rapid cooling.
Here, in the first rapid cooling step, the water amount density of the cooling water in the first rapid cooling device 4 is set to 1.0 m ³ / (min · m ² ) or more and 4.0 m ³ / (min · m ² ) or less and the Cooling water is jetted from the cooling nozzles 6a and 6b to the thick steel plate S to perform rapid cooling.
Further, in the first rapid cooling step, the cooling water 12 from the upper rapid cooling nozzle 6a of the first rapid cooling device 4 is restrained by the drainer 9 to prevent the leakage of the cooling water to the outside of the cooling section.

Further, in the first rapid cooling step, the control device 10 causes the plate steel sheet S to have a target temperature within the range of 550 ° C. to 800 ° C. at the plate thickness cross-section average temperature at the end point of the first rapid cooling device 4, and will be described later. In the second rapid cooling step of No. 2, on the assumption that the plate thickness cross-section average temperature of the thick steel plate S at the end point of the second rapid cooling device 5 is stopped at the target temperature within the range of room temperature to 550 ° C. In the first rapid cooling device 4 for injecting cooling water, the transport speed of the thick steel plate S in the first rapid cooling device 4 and the transport speed of the thick steel plate S in the second rapid cooling device 5 are the same. The number of the rapid cooling nozzles 6a and 6b (logarithm of the rapid cooling nozzles 6a and 6b), the water density of the cooling water in the first rapid cooling device 4, the rapid cooling nozzles 7a and 7b in the second rapid cooling device 5 for injecting the cooling water. Number (logarithm of quenching nozzles 7a, 7b), second quench Calculated water amount density of the cooling water in the retirement unit 5, and the conveying speed of the steel plate S in the first rapid cooling device 4 and the second rapid cooling device 5. However, regarding the water amount density of the cooling water in each of the first rapid cooling device 4 and the second rapid cooling device 5, as described above, 1.0 m ³ / (min · m ² ) or more and 4.0 m ³ / (min・ M ² ) or less.
Then, in the first rapid cooling step, the control device 10 determines the thickness based on the calculated number of the rapid cooling nozzles 6a and 6b in the first rapid cooling device 4 and the water amount density of the cooling water in the first rapid cooling device 4. While cooling the steel plate S, the thick steel plate S is transported at the calculated transport speed.

Next, in the cooling process, the thick steel plate S rapidly cooled in the first quenching process is placed in a position where the distance between the thick steel plate S and the first quenching device 4 of the cooling device 3 is 5 m or more. Before being cooled by the second rapid cooling device 5, the air cooling is waited (air cooling waiting step).
Here, the control device 10 determines the ferrite fraction targeted by the structure of the thick steel plate S based on the measurement value by the thermometer 20 that measures the surface temperature of the thick steel plate S after the cooling by the first rapid cooling device 4. The air cooling standby time in front of the second rapid cooling device 5 is calculated so that the thick steel plate S exiting from the first rapid cooling device 4 has the second rapid cooling device 5 after the calculated air cooling standby time has elapsed. The transport speed of the thick steel plate S is controlled so as to be cooled by.

Then, in the cooling step, the thick steel plate S that has been air-cooled in the air-cooling standby step is rapidly cooled by the second rapid cooling device 5 (second rapid cooling step).
In the second rapid cooling step, cooling water is supplied from the rapid cooling nozzles 7a and 7b of the second rapid cooling device 5 in which a plurality of pairs of upper and lower rapid cooling nozzles 7a and 7b are arranged side by side along the transport direction of the thick steel plate S. It is injected into S to perform rapid cooling.
Here, in the second rapid cooling step, the water amount density of the cooling water in the second rapid cooling device 5 is set to 1.0 m ³ / (min · m ² ) or more and 4.0 m ³ / (min · m ² ) or less to achieve the rapid cooling. Cooling water is jetted from the cooling nozzles 7a and 7b to the thick steel plate S to perform rapid cooling.

In the second rapid cooling step, the cooling water 12 from the upper rapid cooling nozzle 7a of the second rapid cooling device 5 is restrained by the drainer 9 to prevent the leakage of the cooling water to the outside of the cooling section.
Further, in the second rapid cooling step, the control device 10 controls the number of the rapid cooling nozzles 7a and 7b in the second rapid cooling device 5 calculated above and the water amount density of the cooling water in the second rapid cooling device 5. While cooling the thick steel plate S, the thick steel plate S is transported at the calculated transport speed.
Then, the thick steel plate S that has undergone the cooling step is subjected to a post step.

  As described above, according to the method for manufacturing the thick steel plate S according to the second embodiment, the thick steel plate S extracted from the heating furnace 2 has a plate thickness cross-section average temperature of 550 in the first rapid cooling step. After rapid cooling to ˜800 ° C., air cooling standby is performed in the air cooling standby process to first generate ferrite in a predetermined fraction. After that, rapid cooling is performed in the second rapid cooling step to stop the thick steel sheet S at a target temperature in the range of room temperature to 550 ° C. at the end point of the second rapid cooling device 5, where the average sheet thickness cross-section temperature is The remaining austenite phase is the bainite phase or martensite phase. As a result, the ferrite fraction of the first phase is controlled and the bainite phase or martensite phase of the second phase is formed in one heat treatment step. As a result, it is possible to disperse the soft ferrite phase and the hard phase bainite phase or martensite phase as a structure of the thick steel plate in an appropriate ratio according to the strength, and achieve a low yield ratio while maintaining the tensile strength. be able to. Since the thick steel sheet S having a low yield ratio can be obtained in one heat treatment step, the energy cost can be reduced by manufacturing the steel sheet S with the least possible energy.

Although the embodiment of the present invention has been described above, the present invention is not limited to this, and various modifications and improvements can be made.
For example, the water draining device 9 is provided between the upper rapid cooling nozzles 6a adjacent to each other in the transport direction of the first rapid cooling device 4 and on the upstream side and the uppermost rapid cooling nozzles on the upstream side of the uppermost rapid cooling nozzle 6a located at the uppermost stream. A draining device 9 is arranged on the downstream side of 6a. Similarly, in the second rapid cooling device 5 as well, it is located between the upper rapid cooling nozzles 7a adjacent to each other in the transport direction of the second rapid cooling device 5 and on the upstream side and the downstream side of the uppermost rapid cooling nozzle 7a located at the uppermost stream. A drainer 9 is arranged downstream of the upper quench nozzle 7a. However, the draining device 9 includes the upper rapid cooling nozzles 6a located at the uppermost stream and the uppermost downstream rapid cooling nozzles 6a and 7a at least in the first rapid cooling device 4 and the second rapid cooling device 5, respectively. , 7a, and it is not always necessary to arrange them between the upper rapid cooling nozzles 6a and between the upper rapid cooling nozzles 7a.

  In order to verify the effect of the present invention, in a heat treatment facility 1 shown in FIG. 10, after heating a thick steel plate (plate thickness 15 mm × plate width 3000 mm × plate length 15 m) at room temperature to 910 ° C. in a heating furnace 2, Cooling was carried out under the cooling conditions of Inventive Examples 1 to 8 and Comparative Examples 1 to 3, and low yield ratio thick steel sheets were manufactured with the targets of tensile strength of 490 MPa or more and 600 MPa or less and yield ratio of 80% or less. The yield ratio was 80% or less and passed (∘), and if it was greater than 80%, it was rejected (x), but it was manufactured so that the target yield ratio was 75%. Further, in order to manufacture a thick steel plate with a small variation in material, it is necessary to suppress the temperature deviation within the steel plate surface on the outlet side of the second rapid cooling device 5 to 30% or less, and the temperature deviation is 30% or less. The samples were evaluated as passing (◯), and those exceeding 30% were rejected (x). The temperature deviation within the steel plate surface was evaluated as a value obtained by subtracting the minimum value from the maximum value among the steel plate surface temperatures measured on the entire surface of the thick steel plate using a scanning radiation thermometer. The material test was carried out by collecting samples from a part of the tip and the tail of the thick steel plate.

In the heat treatment equipment 1 shown in FIG. 10, the distance between the first quenching device 4 and the second quenching device 5 is 5 m, spray cooling is used as the quenching nozzles 6a, 6b, 7a, 7b, and water is drained. A draining roll was installed as the device 9. Further, a scanning radiation thermometer capable of measuring the temperature distribution in the plane of the steel plate was installed above the inlet side and the outlet side of the first and second rapid cooling devices 4 and 5. Further, in order to secure the target material, the control device 10 rapidly sprays the cooling water so that the plate thickness cross-section average temperature of the thick steel plate at the end point of the first rapid cooling device 4 becomes 700 ° C. The number of cooling nozzles 6a and 6b, the water amount density of the cooling water in the first rapid cooling device 4, and the transport speed of the thick steel plate in the first rapid cooling device 4 were controlled. Further, the control device 10 controls the number of the rapid cooling nozzles 7a, 7b for injecting the cooling water so that the average thickness of the thick steel sheet at the end of the second rapid cooling device 5 at the end point of the second rapid cooling device 5 is 400 ° C. The water density of the cooling water in the rapid cooling device 5 and the transport speed of the thick steel plate in the second rapid cooling device 5 were controlled. The distance from the heating furnace 2 to the first quenching device 4 was 4 m.
The results are shown in Table 1.

In Example 1 of the present invention, after the thick steel plate is rapidly cooled by the first rapid cooling device 4, air cooling is put on standby to generate ferrite, and then bainite is generated by the second rapid cooling device 5, which is 80% or less ( A thick steel plate having a yield ratio of 74% at the tip and 73% at the tail could be obtained. Further, the temperature deviation in the steel plate surface of the thick steel plate obtained in Inventive Example 1 was 25 ° C., which was acceptable.
On the other hand, in Comparative Example 1, the thick steel plate was finished by the rapid cooling by the first rapid cooling device 4, and thereafter was air-cooled to room temperature without being rapidly cooled by the second rapid cooling device 5. As a result, since the structure of the thick steel sheet did not form ferrite and became a bainite + martensite structure, a thick steel sheet having a yield ratio of 85% was formed at the tip and the tail end of the thick steel sheet. On the other hand, the temperature deviation in the steel plate surface of the thick steel plate obtained in Comparative Example 1 was 21 ° C., which was acceptable.

In Example 2 of the present invention, the water amount densities of the cooling water in the first rapid cooling device 4 and the second rapid cooling device 5 are 1.2 m ³ / (min · m ² ), 1.2 m ³ / (min · m ² ), respectively. Therefore, it is possible to cool more uniformly than Example 1 of the present invention, and while achieving a yield ratio of 80% or less (74% at the tip of thick steel plate, 73% at the tail end), temperature deviation in the steel plate plane Was 22 ° C., and a high-quality low yield ratio tempered steel sheet with little material variation was obtained.
Similarly, in the third example of the present invention, the water amount densities of the cooling water in the first rapid cooling device 4 and the second rapid cooling device 5 are 1.5 m ³ / (min · m ² ), 1.5 m ³ / (min), respectively. .M ² ), it can be cooled more uniformly than Example 1 of the present invention, and the yield ratio is 80% or less (74% at the tip of the thick steel plate, 73% at the tail end), while maintaining the steel plate surface. The temperature deviation inside was 19 ° C, and a high-quality low-yield ratio heat-treated steel sheet with little material variation was obtained.

On the other hand, in Comparative Example 2, the water amount density of the cooling water in the first rapid cooling device 4 is as small as 0.9 m ³ / (min · m ² ), and in Comparative Example 3, the cooling water in the second rapid cooling device 5 is Has a small water amount density of 0.9 m ³ / (min · m ² ) and is slowly cooled, so transitional boiling occurs during cooling and the temperature deviations in the steel sheet surface are 34 ° C. and 33 ° C., respectively. It was a big failure.
Further, in the invention example 4, the air cooling standby time at the tip of the thick steel plate is controlled based on the measured value of the steel plate surface temperature at the end of the rapid cooling by the first rapid cooling device 4, so the ferrite fraction is accurately controlled. The yield ratio at the tip of the thick steel plate could be set to 75% of the target.

Further, in Example 5 of the present invention, since the transport speed control was performed so that the transport speed of the thick steel plate in the first rapid cooling device 4 and the transport speed of the thick steel plate in the second rapid cooling device 5 were the same, the heating furnace 2 The temperature history from extraction to the end of cooling can be made uniform over the entire length of the thick steel plate, and the yield ratio becomes 74% at the tip and the tail end of the thick steel plate, and the yield ratio in the entire surface of the thick steel plate, especially in the longitudinal direction It was possible to manufacture a low yield ratio heat-treated steel sheet with a small variation in.
Further, in Inventive Example 6, since both the air cooling standby time control and the conveyance speed control were performed, the ferrite fraction was accurately controlled, and the temperature history from the extraction of the heating furnace 2 to the end of cooling was measured over the entire length of the thick steel plate. Since it was possible to align, the yield ratio at the tip and tail end of the thick steel plate was the same as the target 75%, and the low yield ratio tempered steel plate with little variation in the yield ratio over the entire surface of the thick steel plate, especially in the longitudinal direction. Could be manufactured.
Furthermore, in Inventive Examples 7 and 8, since the pressing force of the draining roll was 6 ton and 8 ton, respectively, the draining property was very good, and the temperature deviation in the steel sheet plane was 23 ° C. as compared with Inventive Example 1, respectively. It became as small as 21 ° C.

1 Off-line heat treatment equipment (production equipment for thick steel plates)
2 heating furnace 3 cooling device 4 first quenching device 5 second quenching device 6a upper quenching nozzle 6b lower quenching nozzle 7a upper quenching nozzle 7b lower quenching nozzle 8 table roll 9 drainer 10 Control device (first rapid cooling control device, second rapid cooling control device, air cooling standby time control device)
11 Upper Computer 12 Cooling Water 13 Leakage Water 14 Circular Pipe Jet 15 Purge Water 60a Cooling Header 60b Cooling Header S Steel Plate

Claims (14)

What is claimed is: 1. A steel plate manufacturing facility comprising a heating furnace for heating a steel plate of 100 ° C. or lower to an austenite temperature range, and a cooling device for cooling the steel plate heated by the heating furnace,
The cooling device includes a first rapid cooling device that rapidly cools the thick steel plate extracted from the heating furnace, and a second rapid cooling device that is installed downstream of the first rapid cooling device in the transport direction and that rapidly cools the thick steel plate. Equipped with a cooling device,
In each of the first rapid cooling device and the second rapid cooling device, upper and lower pairs of rapid cooling nozzles are arranged side by side along the transport direction of the thick steel plate, and at least the first rapid cooling device and the second rapid cooling device are arranged. Arranging a water draining device on the upstream side of the uppermost quenching nozzle in the uppermost stream and the downstream side of the uppermost quenching nozzle in the most downstream in each of the cooling devices,
A facility for manufacturing a thick steel plate, characterized in that a distance between the first rapid cooling device and the second rapid cooling device is 5 m or more. Water density of cooling water in each of the first rapid cooling device and the second rapid cooling device is set to 1.0 m ³ / (min · m ² ) or more and 4.0 m ³ / (min · m ² ) or less. The thick steel plate manufacturing facility according to claim 1.   The number of rapid cooling nozzles for injecting cooling water so that the plate thickness cross-section average temperature of the thick steel plate at the end point of the first rapid cooling device becomes a target temperature within the range of 550 ° C. to 800 ° C. The thick steel plate according to claim 2, further comprising a first rapid cooling control device that controls a water amount density of cooling water in the rapid cooling device and a transport speed of the thick steel plate in the first rapid cooling device. Manufacturing equipment.   The number of rapid cooling nozzles for injecting cooling water so that the plate thickness cross-section average temperature of the thick steel plate at the end point of the second rapid cooling device becomes a target temperature within the range of room temperature to 550 ° C., the second rapid cooling nozzle. The thick steel plate according to claim 3, further comprising a second rapid cooling control device that controls a water amount density of cooling water in the cooling device and a transport speed of the thick steel plate in the second rapid cooling device. production equipment.   A thermometer installed between the first rapid cooling device and the second rapid cooling device for measuring the surface temperature of the thick steel plate at the end of cooling by the first rapid cooling device, and a measurement value by the thermometer. Based on the above, the air cooling standby time before the second rapid cooling device for calculating the target ferrite fraction in the structure of the thick steel plate is calculated, and at the same time, the thick steel plate exiting the first rapid cooling device is calculated. 5. An air-cooling standby time control device that controls the transport speed of the thick steel plate so that the second rapid cooling device cools after the elapse of the air-cooling standby time that has been set. Manufacturing equipment for the thick steel plate described.   In the first rapid cooling control device and the second rapid cooling control device, the plate thickness cross-section average temperature of the thick steel plate at the end point of the first rapid cooling device is a target temperature within a range of 550 ° C to 800 ° C. And, on the premise that the plate thickness cross-section average temperature of the thick steel plate at the end point of the second rapid cooling device is stopped at a target temperature within the range of room temperature to 550 ° C., the first rapid cooling is performed. The number of rapid cooling nozzles in the first rapid cooling device that injects cooling water such that the transport speed of the thick steel plate in the device and the transport speed of the thick steel plate in the second rapid cooling device are the same, 1. Water density of cooling water in the quenching device, number of quenching nozzles in the second quenching device for injecting cooling water, water density of cooling water in the second quenching device, and first quenching device, Of the thick steel plate in the second quenching device The feed rate is calculated, and the calculated number of the rapid cooling nozzles in the first rapid cooling device, the water density of the cooling water in the first rapid cooling device, the number of the rapid cooling nozzles in the second rapid cooling device, and the In the state where the thick steel plate is cooled with the water quantity density of the cooling water in the second rapid cooling device, the thick steel plate is conveyed at the calculated transportation speed in the first rapid cooling device and the second rapid cooling device at the same speed. It conveys, The manufacturing equipment of the thick steel plate of Claim 4 or 5 characterized by the above-mentioned.   The thick steel plate having a temperature of 100 ° C. or lower has scales on the surface of the steel plate removed by a scale removing mechanism before heating in the heating furnace, and the distance from the heating furnace to the first rapid cooling device is 4 m or less. The thick steel sheet manufacturing facility according to any one of claims 1 to 6. A method for producing a thick steel sheet comprising: a heating step of heating a thick steel sheet at 100 ° C. or lower in a heating furnace to an austenite temperature range; and a cooling step of cooling the thick steel sheet heated in the heating step with a cooling device,
The cooling step includes a first rapid cooling step of rapidly cooling the thick steel sheet extracted from the heating furnace by a first rapid cooling device of the cooling device, and a thick steel sheet rapidly cooled in the first rapid cooling step. An air-cooling standby step of waiting for air-cooling before cooling by the second rapid-cooling device of the cooling device, which is installed at a position where the distance between the cooling device and the first rapid cooling device is 5 m or more, A second rapid cooling step of rapidly cooling the thick steel sheet that has been air-cooled in the standby step by the second rapid cooling device;
In each of the first rapid cooling step and the second rapid cooling step, a plurality of pairs of upper and lower rapid cooling nozzles of the first rapid cooling device and the second rapid cooling device in which they are arranged side by side along the conveyance direction of the thick steel plate are arranged. Performs rapid cooling by spraying cooling water from the respective rapid cooling nozzles onto the thick steel plate,
At least a water draining device is arranged on the upstream side of the uppermost rapid cooling nozzle on the uppermost stream and on the downstream side of the uppermost rapid cooling nozzle on the most downstream side in each of the first rapid cooling device and the second rapid cooling device. The cooling water is prevented from leaking to the outside of the cooling section by restraining the cooling water from the upper quenching nozzle of each of the first and second quenching devices by the draining device. Manufacturing method of thick steel plate. In each of the first rapid cooling step and the second rapid cooling step, the water amount density of the cooling water in each of the first rapid cooling device and the second rapid cooling device is 1.0 m ³ / (min · m ² ) Or more and 4.0 m < ³ > / (min * m < ² >) or less, cooling water is sprayed from the said rapid cooling nozzle to the said thick steel plate, and rapid cooling is performed, The thick steel plate manufacturing method of Claim 8 characterized by the above-mentioned. ..   The first rapid cooling controller injects cooling water so that the average thickness of the thick steel sheet at the end point of the first rapid cooling device becomes a target temperature in the range of 550 ° C to 800 ° C. The method for manufacturing a thick steel plate according to claim 9, wherein the number of cooling nozzles, the water density of the cooling water in the first quick cooling device, and the transport speed of the thick steel plate in the first rapid cooling device are controlled. ..   Rapid cooling by injecting cooling water by the second rapid cooling control device so that the average thickness of the thick steel plate at the end point of the second rapid cooling device becomes the target temperature within the range of room temperature to 550 ° C. The method for manufacturing a thick steel plate according to claim 10, wherein the number of nozzles, the water density of the cooling water in the second quick cooling device, and the transport speed of the thick steel plate in the second rapid cooling device are controlled.   The thickness is based on a value measured by a thermometer installed between the first rapid cooling device and the second rapid cooling device and measuring the surface temperature of the thick steel plate after cooling by the first rapid cooling device. The air cooling standby time in front of the second rapid cooling device for calculating the target ferrite fraction in the structure of the steel plate is calculated, and the thick steel plate exiting the first rapid cooling device is calculated for the air cooling standby. The method for manufacturing a thick steel sheet according to claim 11, wherein the transport speed of the thick steel sheet is controlled so that the second rapid cooling device cools the steel sheet after a lapse of time.   In the first rapid cooling control device and the second rapid cooling control device, the plate thickness cross-section average temperature of the thick steel plate at the end point of the first rapid cooling device is a target temperature within a range of 550 ° C to 800 ° C. And, on the premise that the plate thickness cross-section average temperature of the thick steel plate at the end point of the second rapid cooling device 5 is stopped at a target temperature within the range of room temperature to 550 ° C., the first rapid cooling is performed. The number of rapid cooling nozzles in the first rapid cooling device that injects cooling water such that the transport speed of the thick steel plate in the cooling device and the transport speed of the thick steel plate in the second rapid cooling device are the same, Water quantity density of cooling water in the first quenching apparatus, number of quenching nozzles in the second quenching apparatus for injecting cooling water, water quantity density of cooling water in the second quenching apparatus, and first quenching apparatus And thick steel plate in the second quenching device A transport speed is calculated, and the calculated number of rapid cooling nozzles in the first rapid cooling device, the amount density of cooling water in the first rapid cooling device, the number of rapid cooling nozzles in the second rapid cooling device, and the In the state where the thick steel plate is cooled with the water quantity density of the cooling water in the second rapid cooling device, the thick steel plate is conveyed at the calculated transportation speed in the first rapid cooling device and the second rapid cooling device at the same speed. It conveys, The manufacturing method of the thick steel plate of Claim 11 or 12 characterized by the above-mentioned.   The thick steel plate having a temperature of 100 ° C. or lower has scales on the surface of the steel plate removed by a scale removing mechanism before heating in the heating furnace, and the distance from the heating furnace to the first rapid cooling device is 4 m or less. The method for manufacturing a thick steel plate according to any one of claims 8 to 13, wherein:

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