JP7452696B2 - Manufacturing method and manufacturing equipment for thick steel plates - Google Patents

Description

The present invention relates to a method and equipment for manufacturing thick steel plates.

In recent years, as building structures have become larger, there has been a demand for higher strength steel materials used. At the same time, from the viewpoint of safety, it is also required to have a high allowable stress and to reduce the yield ratio, which is the ratio of yield strength to tensile strength. Reducing the yield ratio increases the stress and uniform elongation that can be tolerated before failure even when stress exceeding the yield point is applied, resulting in a steel material with excellent plastic deformability, which is suitable for building structures. be.

The basic technical concept of manufacturing low yield ratio steel is as follows. A composite structure of ferrite phase and bainite or martensite phase is built in, and the soft ferrite phase keeps the yield stress low. By obtaining high tensile strength with a hard bainite or martensitic phase, the yield ratio is lowered.

A composite structure is generally obtained mainly by controlling cooling, especially accelerated cooling immediately after hot rolling. More specifically, the accelerated cooling is divided into two stages: slow cooling in the first stage and rapid cooling in the second stage, and the soft ferrite phase is sufficiently grown in the slow cooling in the first stage. Then, by obtaining a hard bainite or martensitic phase in the subsequent rapid cooling, a composite structure that achieves a low yield ratio is obtained.

For example, Patent Document 1 describes a method for producing a steel pipe with a low yield ratio and high weldability, which is composed of a composite structure including a soft ferrite phase and a hard bainite or martensitic phase. According to the method for manufacturing a low yield ratio welded steel pipe disclosed in Patent Document 1, a steel plate immediately after hot rolling is slowly cooled until its temperature reaches around 600°C, and then a post-cooling process is performed, in which the steel plate is rapidly cooled to the coiling temperature. Accelerated cooling is carried out in two stages. The first stage of slow cooling allows the soft ferrite phase to grow sufficiently, and the second stage of rapid cooling produces the hard bainite or martensite phase.

Furthermore, according to Patent Document 1, when steel plates are welded to form a steel pipe, and if the wall thickness of the steel pipe is t and the outer diameter is D, then when t/D≦2, the yield ratio≦80%, and when 2<t/D It is said that a low yield ratio welded steel pipe can be obtained which satisfies the yield ratio ≦85% when t/D>3 and the yield ratio ≦88% when t/D>3. As a specific cooling rate for precipitating a soft ferrite phase, Patent Document 2 discloses a technique in which a thick steel plate with a thickness of 25 mm is subjected to slow cooling at a cooling rate of 5 to 15° C./s.

Figure 2 shows an example of the history of the surface temperature of a thick steel plate during the water-cooling process of a high-temperature thick steel plate. As shown in FIG. 2, at the initial stage of water cooling, the thick steel plate is cooled in a film boiling state where there is a vapor film between the water and the thick steel plate. In the film boiling state, water does not come into direct contact with the steel plate, so the heat transfer coefficient, which is an indicator of cooling capacity, is low and the surface temperature decreases slowly.

However, when the surface temperature reaches about 700 to 500° C., it becomes difficult to maintain a vapor film between the water and the steel plate, and the steel plate is cooled in a transition boiling state where the water and the steel plate partially contact each other. Once water comes into contact with the steel plate, the water that has come into contact with the steel plate evaporates, causing the water to flow in the vicinity of the steel plate more intensely, causing a rapid increase in heat transfer coefficient and a rapid drop in surface temperature. Thereafter, the steel plate enters a nucleate boiling state in which contact with water occurs regularly while maintaining a high heat transfer coefficient, and the surface temperature rapidly drops to near the water temperature.

When slowly cooling a thick steel plate to around 600°C as in the technology described in Patent Document 1 or Patent Document 2, it is preferable to cool the thick steel plate in a film boiling state with a low heat transfer coefficient. be. However, as mentioned above, when the surface temperature of the thick steel plate reaches approximately 700 to 500°C, the film boiling state transitions to the nucleate boiling state where the heat transfer coefficient is high. Once the state of nucleate boiling occurs, the surface temperature of the thick steel plate drops rapidly and the cooling rate becomes excessive, making it impossible to manufacture a thick steel plate with desired properties. In addition, if a transition to a nucleate boiling state occurs in a part of a thick steel plate, it is not possible to build in a predetermined material only in that part, and it is not possible to obtain homogeneous properties over the entire surface of the thick steel plate. Therefore, when water-cooling a thick steel plate in a film boiling state, it is important to control the temperature at which the film boils to transition to a nucleate boiling state and to stably maintain the film boiling state.

Note that from the schematic diagram of the water cooling process shown in FIG. 2, it can be seen that the film boiling state always transitions to the nucleate boiling state. Regarding the lower limit value, that is, the lower limit value of the transition temperature, for example, in Non-Patent Document 1, it is considered that the spontaneous nucleation temperature can be used. In other words, it is said that the interface temperature between the refrigerant and the thick steel plate exceeding the spontaneous nucleation temperature of the refrigerant is a necessary condition for evaporation nuclei to be generated in the refrigerant and the refrigerant to transition to a film boiling state.

The interfacial temperature Tb between the refrigerant and the thick steel plate is expressed by equation 1, where λ is the thermal conductivity, α is the thermal diffusivity, and T is the temperature. shown.

When the refrigerant is water, Tb is approximately 300°C, so if the water temperature Tw is 30°C, the steel plate temperature Ts at which spontaneous nucleation occurs in water when water cooling carbon steel is approximately 330~300°C. It becomes 350℃. Therefore, the film boiling state can only be physically maintained at a temperature of 330 to 350°C at the lowest, and it is necessary to stop film boiling cooling when the surface layer temperature of the thick steel plate exceeds this temperature. Therefore, it should be noted that cooling water should be injected onto the thick steel plate before the temperature of the front and back layers of the thick steel plate reaches at least 350°C.

If the required cooling rate is high, this can be achieved by increasing the transition temperature and intermittent nucleate boiling and air cooling. For example, in Patent Document 3, cooling nozzles arranged in a row in the transport direction of a thick steel plate are separated by pinch rolls, and rapid cooling to a nucleate boiling state is achieved by injecting a large flow of water, and air cooling without injecting water. The states are set alternately. It is said that this allows stable cooling at a surface layer cooling rate of 30° C./s or more.

As a cooling method that does not involve the boiling transition as described above, it is conceivable to inject gas (for example, air) onto the thick steel plate. However, in general, the cooling capacity of forced convection of gas is one order of magnitude lower than the film boiling cooling capacity of water cooling, and it is necessary to inject gas at high speed in order to obtain the target cooling rate. Therefore, it is necessary to inject the gas after compressing it with a compressor or the like, but this is not preferable since the manufacturing cost increases due to the power consumption of the compressor.

Considering the above circumstances, when manufacturing a product in which a composite structure is created by dividing low yield ratio steel into a slow cooling stage in the first stage and a rapid cooling stage in the latter stage, it is necessary to perform the slow cooling in the first stage in a film boiling state with a low heat transfer coefficient. Water cooling is preferred. However, the film boiling state transitions to the nucleate boiling state where the heat transfer coefficient is high when the surface temperature of the thick steel plate reaches 700 to 500°C, so it is important to maintain the film boiling state stably down to low temperatures. .

As a technique for stably maintaining a film boiling state down to a low temperature, for example, Patent Document 4 discloses a technique for reducing the transition temperature to a nucleate boiling state by raising cooling water to a high temperature. Further, Patent Document 5 discloses a technique of increasing the heat transfer coefficient in the film boiling state while reducing the transition temperature to the nucleate boiling state by adding steam to water.

Japanese Patent Application Publication No. 10-17980 Japanese Patent Application Publication No. 2005-313223 Japanese Patent Application Publication No. 2005-154841 Japanese Unexamined Patent Publication No. 58-71339 Japanese Patent Application Publication No. 10-300301

Power Reactor and Nuclear Fuel Development Corporation, “Steam explosion phenomena related to nuclear reactor safety evaluation”, February 1980

However, in the technique described in Patent Document 2, there is no mention of a specific method for maintaining weak cooling of 5 to 15°C/s to a sufficiently low temperature, and the lower limit thereof is up to 600°C. Considering variations in processes and components, it is thought that slow cooling to a low temperature of 600° C. or lower is necessary, and therefore, it is thought that the characteristics of the manufactured product will not be stable with the technique described in Patent Document 2.

In the technique disclosed in Patent Document 3, cooling by alternately providing rapid cooling by nucleate boiling and air cooling is limited to conditions where the surface layer cooling rate is 30° C./s or more. Therefore, the technology of Patent Document 3 is suitable for low cooling speed processes where the surface layer cooling rate is less than 30°C/s, such as the process with a cooling rate of 5 to 15°C/s required by the technology of Patent Document 5. Not applicable.

As a technique for reducing the transition temperature, in order to raise the water temperature as in Patent Document 4, incidental equipment such as a heater is required, and the running cost thereof increases manufacturing costs. In addition, when cooling water for thick steel plates is recycled, the temperature of the cooling water fluctuates depending on the extraction efficiency of the steel plates, making its management even more difficult.

Further, even when steam is used as in Patent Document 5, additional equipment for generating steam is required. In addition, it is necessary to install new valves and headers to control the amount of steam, which increases equipment costs and increases the number of items to be managed, leading to deterioration in maintainability. In addition, in both prior art techniques, the transition temperature from film boiling to nucleate boiling is reduced by increasing the cooling water temperature. However, consuming extra energy to raise the water temperature is not preferable from the viewpoint of energy conservation.

By the way, it is generally known that there is a correlation between the water density of cooling water in the cooling zone and the cooling capacity. Therefore, when cooling a thick steel plate at a desired cooling rate, the cooling capacity is adjusted by controlling the water density of the cooling water in the zone. On the other hand, the water density of the cooling water in the cooling zone also has a correlation with the surface temperature of the thick steel plate at the moment of transition from the film boiling state to the nucleate boiling state (hereinafter referred to as transition temperature). Therefore, in the prior art, when there is a desired cooling rate, it is not possible to reduce the water volume density for the purpose of lowering the transition temperature, and it is not possible to independently lower only the transition temperature.

Therefore, as described in Non-Patent Document 1, although a physical lower limit temperature that can maintain cooling in the film boiling state has been proposed, there is no method to reduce the boiling transition temperature with the water density parameter fixed. unknown.

The present invention has been made in view of the above circumstances, and its purpose is to provide a method for manufacturing thick steel plates that maintains a film boiling state down to low temperatures without adding additional equipment or consuming excess energy. The purpose is to provide facilities.

The gist of the present invention for solving the above problems is as follows.
[1] A method for manufacturing a thick steel plate in which the thick steel plate is water-cooled, using a water cooling device having a plurality of sets of at least one pair of upper and lower cooling water injection nozzles arranged along the conveyance direction of the thick steel plate. A method for manufacturing a thick steel plate, wherein the thick steel plate is cooled by water at a cooling water injection speed of a cooling water injection nozzle of 0.4 m/s or more and 30 m/s or less.
[2] The method for manufacturing a thick steel plate according to [1], wherein the surface cooling rate of the thick steel plate in the water cooling device is 0.4°C/s or more and 29°C/s or less.
[3] The method for manufacturing a thick steel plate according to any one of [1] or [2], wherein the thick steel plate is descaled before the water cooling.
[4] The method for producing a thick steel plate according to any one of [1] to [3], wherein the thick steel plate is subjected to oxide scale removal treatment and heating before the water cooling.
[5] The method for manufacturing a thick steel plate according to any one of [1] to [4], wherein the temperature of one or both of the upper and lower surfaces of the thick steel plate is measured after the water cooling.
[6] The method for manufacturing a thick steel plate according to any one of [2] to [5], wherein the surface cooling rate is controlled by controlling the amount of cooling water injected from the cooling water injection nozzle.
[7] Controlling the cooling stop temperature of the thick steel plate by controlling the number of the cooling water injection nozzles and the conveyance speed of the thick steel plate, and setting the cooling stop temperature to a surface temperature of the thick steel plate of 350°C or higher. The method for producing a thick steel plate according to any one of [1] to [6].
[8] A thick steel plate production facility that performs water cooling of thick steel plates, the water cooling device having a plurality of sets of at least one pair of upper and lower cooling water injection nozzles arranged along the conveying direction of the thick steel plate, and the cooling water A thick steel plate manufacturing facility, comprising: a control device that controls the injection speed of cooling water injected from an injection nozzle to 0.4 m/s or more and 30 m/s or less.
[9] The thick steel plate manufacturing equipment according to [8], wherein the control device controls a surface cooling rate of the thick steel plate within the water cooling device to 0.4° C./s or more and 29° C./s or less.
[10] The thick steel plate manufacturing equipment according to any one of [8] or [9], further comprising a descaling device on the inlet side of the water cooling device.
[11] Any one of [8] to [10], further comprising a thermometer for measuring the temperature of one or both of the upper and lower surfaces of the thick steel plate on the exit side of the water cooling device. Manufacturing equipment for the thick steel plates listed.
[12] The thick steel plate manufacturing equipment according to any one of [8] to [11], wherein the control device further controls the amount of cooling water injected from the cooling water injection nozzle.
[13] The thick steel plate manufacturing equipment according to any one of [8] to [12], wherein the control device controls the number of the cooling water injection nozzles and the conveyance speed of the thick steel plate.

According to the thick steel plate manufacturing equipment and manufacturing method according to the present invention, when a thick steel plate is water-cooled at a significantly low cooling rate, a film boiling state can be stably maintained down to a low temperature. By stably maintaining the film boiling state down to low temperatures, it is possible to produce thick steel plates with uniform properties over the entire surface.

FIG. 1 is a diagram showing a schematic configuration of a heat treatment facility for thick steel plates. It is a graph showing the surface temperature history of a thick steel plate in a water cooling process. It is a graph showing the relationship between the injection speed of cooling water and the transition temperature of a thick steel plate. FIG. 2 is a block diagram showing the configuration of a control device and the like. FIG. 2 is a diagram showing a schematic configuration of a heat treatment facility equipped with a draining roll. 1 is a diagram showing a schematic configuration of a heat treatment facility equipped with a drain purge nozzle. FIG. 2 is a diagram showing a schematic configuration of a heat treatment facility including a draining roll and a draining purge nozzle. FIG. 2 is a diagram showing a schematic configuration of a heat treatment facility that includes large-flow cooling water injection nozzles on the inlet and outlet sides of a water cooling device. FIG. 2 is a diagram showing a schematic configuration of a heat treatment facility including a large-flow cooling water injection nozzle inside a water cooling device. FIG. 1 is a diagram showing a schematic configuration of a heat treatment facility for thick steel plates according to an example. It is a graph which shows the relationship between the water volume density of a cooling zone, and a cooling rate. It is a graph which shows the relationship between the water volume density of a cooling zone, and injection speed.

Embodiments of the present invention will be described below with reference to the drawings. The embodiments shown below illustrate devices and methods for embodying the technical idea of the present invention. It is not limited to the embodiments described below. Furthermore, the drawings are schematic. Therefore, it should be noted that the relationships, ratios, etc. between thickness and planar dimensions are different from those in reality, and the drawings also include portions where the relationships and ratios of dimensions are different.

FIG. 1 is a diagram showing a schematic configuration of a heat treatment equipment for thick steel plates, which is an embodiment of the present invention. As shown in FIG. 1, the heat treatment equipment 1 for thick steel plates, which is an embodiment of the present invention, is an off-line type equipment, which includes an oxide scale removal device (not shown) that performs oxide scale removal treatment on the thick steel plates S; A heating furnace 2 that heats the thick steel plate S to a predetermined temperature, a water cooling device 3 that cools the thick steel plate S heated in the heating furnace 2, and a descaling device that descales the thick steel plate S between the heating furnace 2 and the water cooling device 3. 9. The main components include a thermometer 4 that measures the temperature of the thick steel plate S on the outlet side of the water cooling device 3, and a control device 10 that controls the operation of the water cooling device 3. Further, the heat treatment equipment 1 corresponds to the "thick steel plate manufacturing equipment" in the present invention.

The heating furnace 2 contains a thick steel plate S that has been hot-rolled to a predetermined thickness (for example, 30 mm) and width (for example, 2000 mm) on a hot rolling line located at a location different from the heat treatment equipment 1 and cooled to about room temperature. charged. The thick steel plate S is heated in the heating furnace 2 to a predetermined temperature (for example, 910° C.). The thick steel plate S extracted from the heating furnace 2 is cooled by the water cooling device 3 while being conveyed by a plurality of table rolls 6 installed on the exit side of the heating furnace 2.

Generally, in offline heat treatment equipment, the steel plate is extracted from the heating furnace 2 and transported at a nearly constant speed until it is cooled by the water cooling device 3. is small. That is, if the heating temperature of the thick steel plate is T0, the distance from the heating furnace 2 to the water cooling device 3 is L0, and the conveyance speed of the thick steel plate is V0, the tip of the thick steel plate is extracted at the temperature T0, and the cooling time is L0/ It is cooled down through V0. In offline heat treatment equipment, since the distance L0 from the heating furnace 2 to the water cooling device 3 is short, even if the tip of the thick steel plate is extracted from the heating furnace 2 and reaches the inlet of the water cooling device 3, the tail end of the thick steel plate is The portion is maintained at a temperature T0 in the heating furnace 2. Therefore, the tail end of the thick steel plate is also extracted at the temperature T0 in the same way as the tip, and is cooled after the cooling time L0/V0, so the cooling start temperature can be kept constant over the entire length of the thick steel plate. As described above, offline heat treatment equipment is advantageous in producing thick steel plates that are materially homogeneous over the entire surface of thick steel plates whose temperature tends to drop by cooling.

However, the present invention can also be applied to online type heat treatment equipment. In this case, a thick steel plate that is equipped with heating equipment that heats the thick steel plate, a rolling equipment that rolls the thick steel plate heated by the heating equipment, and a water cooling device that cools the thick steel plate that has been rolled to a predetermined thickness by the rolling equipment. This applies to heat treatment equipment for steel plates. In this case as well, the thick steel plate is heated to a high temperature on the inlet side of the water cooling device, similar to the offline heat treatment equipment. On the other hand, in online heat treatment equipment, the cooling time from immediately after rolling to the start of cooling is longer at the tail end of a thick steel plate than at the leading end. Therefore, when the length of the thick steel plate is L and the conveyance speed of the thick steel plate is v, there will be a difference in cooling time between the tail end and the tip end by the time L/v. Therefore, even if the temperature of the thick steel plate after rolling is uniform, the tail end is allowed to cool for an additional cooling time difference, resulting in a difference in the cooling start temperature of the tip and tail end, which causes characteristics to deteriorate over the entire surface. A uniform thick steel plate cannot be obtained. Therefore, as will be described later, it is preferable that the control device 10 predicts the temperature of the thick steel plate and changes the operating conditions of the water cooling device 3 over the longitudinal position of the thick steel plate S accordingly.

In the heating furnace 2, heating is preferably performed in a non-oxidizing atmosphere (for example, a nitrogen atmosphere). This is because, as will be described later, the transition temperature is affected by the thickness of the oxide scale, and the thicker the oxide scale, the easier the transition from the film boiling state to the nucleate boiling state. At that time, the oxygen concentration in the heating furnace 2 is preferably controlled to 1% (volume %) or less.

The water cooling device 3 includes a water cooling device 3 that cools the thick steel plate S with water under predetermined cooling conditions. The water cooling device 3 includes a plurality of pairs of upper cooling water injection nozzles 32a and lower cooling water injection nozzles 32b, which form pairs above and below with respect to the conveying direction of the thick steel plate S, at a predetermined pitch along the conveying direction of the thick steel plate S. They are arranged side by side. Cooling water 7 is injected toward the thick steel plate S from the cooling water injection nozzles 32 (upper cooling water injection nozzle 32a and lower cooling water injection nozzle 32b). That is, it has a plurality of sets of at least one pair of upper and lower cooling water injection nozzles arranged along the conveyance direction of the thick steel plate S. Further, the thick steel plate S is cooled while being conveyed by table rolls 6 that are arranged side by side at a predetermined pitch along the conveyance direction of the thick steel plate S. Note that the cooling section in which the pair of cooling water injection nozzles 32a and 32b are units is referred to as a cooling zone, and the cooling zone is counted as a "zone." Although FIG. 1 shows a total of seven cooling zones, the effects of the present invention are not impaired even if the cooling zones are other than seven.

The operational parameters of the water cooling device 3 include the amount of cooling water 7 (cooling water amount) injected from the pair of cooling water injection nozzles 32a and 32b, and the conveyance speed of the thick steel plate S conveyed by the table rolls 6. ing. As the amount of cooling water increases, the cooling rate and temperature drop of the thick steel plate S can be increased. On the other hand, the lower the transport speed of the thick steel plate S, the greater the amount of temperature decrease of the thick steel plate S. Furthermore, by combining these operating parameters, the cooling stop temperature and cooling rate can be controlled as cooling conditions to obtain a desired material quality.

As an operating parameter of the water cooling device 3, the balance of the amount of cooling water for each cooling zone (for example, increasing the amount of cooling water in the upstream cooling zone and decreasing the amount of cooling water in the downstream cooling zone, etc.) may be set. . This is because the cooling rate can be controlled according to the temperature range of the thick steel plate S. Furthermore, the number of cooling zones to which cooling water is injected may be set. This is because the cooling stop temperature can be controlled while keeping the cooling rate the same depending on the number of cooling zones used.

It is preferable that the cooling rate can be varied depending on the material, and from the viewpoint of obtaining a soft ferrite phase, the surface layer cooling rate is preferably 29° C./s or less. It is preferable to cool the thick steel plate S at a surface cooling rate of more preferably 15° C./s or less, and still more preferably 10° C./s or less. Furthermore, if the surface layer cooling rate is less than 0.4° C./s, the cooling rate will be almost the same as that of standing cooling, and the production efficiency will decrease. Therefore, the surface layer cooling rate is preferably 0.4° C./s or more.

As mentioned above, from the schematic diagram of the water cooling process shown in FIG. 2, the film boiling state always transitions to the nucleate boiling state, and thereafter the lower limit of temperature is approximately 330°C to 350°C. Therefore, the injection of the cooling water 7 to the thick steel plate S may be stopped when the temperature of the front and back surfaces of the thick steel plate S is preferably 350°C or higher, more preferably 400°C or higher.

Further, the relationship between the cooling water injection speed V and the transition temperature Tt shown in FIG. 3 can be formulated as shown in Equation 2.

Substituting the injection speed for stably injecting cooling water, which will be described later, into this, Tt=350° C. when V=0.4 m/s, and Tt=400° C. when V=0.9 m/s. From this point of view, the injection of the cooling water 7 to the thick steel plate S should preferably be stopped when the temperature of the front and back surfaces of the thick steel plate S is 350°C or higher, more preferably 400°C or higher.

After cooling is stopped at the above temperature, nucleate boiling tends to occur. Therefore, it is preferable to perform rapid cooling using a known technique in a nucleate boiling state. In order to maintain cooling by nucleate boiling, it is preferable that the minimum water density of the cooling zone is 300 L/(m ² ·min) or more. In addition, consideration should be given to ensure that rapid cooling by nucleate boiling is performed even for thick steel plates with high temperature and large heat content, so the minimum water density in the cooling zone is more preferably 1000 L/(m ² · min) or more. . More preferably, it is 1500 L/(m ² ·min) or more, and most preferably 2000 L/(m ² ·min) or more. On the other hand, in the cooling zone, if the water density is greater than 4000 L/(m ² ·min), the cooling rate will hardly change even if the water density is increased. For this reason, it is not preferable from the economic point of view, such as the power of the cooling water, so the maximum water density is preferably 4000 L/(m ² ·min) or less.

Further, as a result of extensive research by the inventors, it has been found that when the water density in the zone is the same, the transition temperature has a correlation with the injection speed of the cooling water injected from the cooling water injection nozzle. FIG. 3 shows the relationship between the injection speed of cooling water injected from the cooling water injection nozzle and the transition temperature. It can be seen from the figure that there is a positive correlation between the cooling water injection speed and the transition temperature. In addition, it can be seen from the figure that when performing slow cooling to 600° C. as in the technique disclosed in Patent Document 1, the injected cooling water should be controlled to 30 m/s or less. Therefore, in order to stably maintain the film boiling state, the injection speed of the cooling water injected from the cooling water injection nozzle 32 is controlled to be 30 m/s or less, preferably 20 m/s or less, and more preferably 7 m/s or less. It should be. Further, in order to stably inject the cooling water 7 from the cooling water injection nozzle 32, the injection speed should be controlled to 0.4 m/s or more, preferably 0.9 m/s or more.

As the cooling water injection nozzle 32, a spray nozzle that can uniformly inject cooling water at a predetermined injection speed can be used. At that time, a spray type in which cooling water is injected while rotating inside a nozzle, specifically, a type such as a full cone spray or a square spray is preferable. This is because the injection speed of the cooling water can be reduced by applying rotational force to the water inside the nozzle. A slit-type nozzle, a multi-hole jet nozzle, or a mist nozzle may be used as long as the injection speed can be sufficiently reduced. Further, it is preferable that the cooling nozzle is capable of changing the cooling water amount density in various ways depending on the target cooling rate.

Note that the operating conditions of the cooling water injection nozzle 32 are not limited to the above. That is, the cooling water injection nozzle 32 is used as a nozzle for both film boiling state cooling and nucleate boiling state cooling, and depending on the desired material, it can perform film boiling state cooling under the above conditions and nucleate boiling state cooling by injecting a large flow of cooling water. Cooling of the boiling state may be used separately.

It is generally known that the transition temperature is influenced by oxide scale generated on the front and back surfaces of the thick steel plate S. Since the thicker the oxide scale, the higher the transition temperature tends to be, in order to stably maintain the film boiling state, it is preferable to remove the oxide scale from the front and back surfaces of the thick steel plate S. Therefore, it is preferable to remove the oxide scale generated on the front and back surfaces of the thick steel plate S using an oxide scale removal device before the steel plate S is charged into the heating furnace 2. Note that as an oxide scale removing device, a shot blasting device may be used to perform shot blasting on the thick steel plate S to remove oxide scale on the front and back surfaces. Alternatively, the thick steel plate S may be pickled using a pickling device to remove oxidized scale on the front and back surfaces. Further, the thick steel plate S may be ground using a grinding device to remove oxidized scale on the front and back surfaces.

Regarding the thickness of oxide scale, typical mill scale is about 10 to 50 μm. By subjecting the thick steel plate S having such a thick oxide scale to the oxide scale removal treatment mentioned as a specific example above, the oxide scale can be reduced to a thickness of less than 1 μm. Therefore, the thickness of the oxide scale covering the front and back surfaces of the thick steel plate S charged into the heating furnace 2 is preferably less than 1 μm.

Note that the oxide scale removal device does not necessarily need to be placed in the same line as the heat treatment equipment 1, and an oxide scale removal device placed in another line, another equipment, or another factory may be used. This is because the degree of freedom in distribution of the thick steel plates S charged into the heating furnace 2 can be increased, and production efficiency can be improved.

The thick steel plate S extracted from the heating furnace 2 is exposed to the atmosphere in a high temperature state. Therefore, while being transported from the heating furnace 2 to the water cooling device 3, oxide scale is generated on the front and back surfaces of the thick steel plate S. Therefore, it is preferable to dispose a descaling device 9 between the heating furnace 2 and the water cooling device 3 to descale the oxide scale attached to the front and back surfaces of the thick steel plate S.

Furthermore, both the descaling device 9 and the oxide scale removing device may be arranged. The oxide scale generated on the thick steel plate S before being charged into the heating furnace 2 is removed by an oxide scale removal device, and the oxide scale generated on the thick steel plate S extracted from the heating furnace 2 is removed by a descaling device 9. , scale can be removed uniformly and easily.

Regarding the thickness of oxide scale, typical mill scale is about 10 to 50 μm. By performing descaling treatment on the thick steel plate S having such a thick oxide scale, the oxide scale can be reduced to a thickness of less than 1 μm. Therefore, the thickness of the oxide scale covering the front and back surfaces of the thick steel plate S entering the water cooling device 3 is preferably less than 1 μm.

As described above, the thick steel plate S passes through the water cooling device 3 so as to stop at a temperature of 350° C. or higher. Therefore, by installing a thermometer 4 on the outlet side of the water cooling device 3 and measuring the surface temperature of the thick steel plate S cooled by the water cooling device 3, it is possible to check whether the thick steel plate S is being cooled as expected. Can be done. In addition, by combining the heating temperature information and the temperature measurement results, performing calculations and heat transfer simulations, and calculating the cooling rate during water cooling, it is also possible to check whether the thick steel plate S is being cooled as expected. Furthermore, it may be confirmed whether the thick steel plate S is uniformly cooled by measuring the in-plane temperature distribution of the thick steel plate S after water cooling.

The thermometer 4 is a device that measures the temperature of the thick steel plate S by scanning the thermometer in the width direction of the thick steel plate S or by arranging one or more thermometers in the width direction of the thick steel plate S. . Further, the thermometer 4 measures the temperature of one or both of the upper and lower surfaces of the thick steel plate.

In FIG. 1, the thermometer 4 is installed on the outlet side of the water cooling device 3, but the thermometer 4 may be installed inside the water cooling device 3 as long as it can measure the temperature of the thick steel plate after cooling by the water cooling device 3. At that time, a plurality of thermometers 4 may be arranged in parallel in the conveying direction of the thick steel plate S, and the temperature of the thick steel plate S in each cooling zone may be measured. Furthermore, a thermometer 4 may be installed on the inlet side of the water cooling device 3 to measure the heating temperature and cooling start temperature of the thick steel plate S. This is because measuring the temperature of the thick steel plate S at the entrance side of the water cooling device 3 improves the accuracy of calculating the cooling rate.

The control device 10 is configured by a known information processing device such as a personal computer. The control device 10 receives size information such as the heating temperature and plate thickness of the thick steel plate S from the host computer 11, as well as the target range of the cooling stop temperature (target cooling stop temperature) necessary to obtain the desired material quality and the cooling rate. Obtain information about the target range (target cooling rate). Then, the control device 10 calculates the operating conditions of the heat treatment equipment 1 to realize such conditions, and determines the operating parameters of each device of the water cooling device 3.

In this embodiment, as shown in FIG. 4, the control device 10 functions as the water cooling condition calculating section 12 by executing a computer program. The water cooling condition calculation unit 12 calculates heat transfer based on an internal model, and determines the number of cooling zones to be used, the amount of cooling water, and the thickness of the steel plate S so as to satisfy the target cooling stop temperature and target cooling rate set as the cooling conditions. Determine the transport speed. The command values for the amount of cooling water and the transport speed of the thick steel plate S thus determined are sent from the water cooling operation condition output section 13 to the water cooling device 3. In the water cooling device 3, based on the command values of the amount of cooling water and the conveyance speed of the thick steel plate S, the operating pressure and number of operating cooling water pumps, the number of headers provided upstream of the cooling water injection nozzle 32, and the flow rate adjustment valve are determined. The opening degree of the table roll 6 and the rotational speed of the motor that drives the table roll 6 are determined.

Next, a method for producing a thick steel plate according to the present invention using the heat treatment equipment 1 shown in FIG. 1 will be explained. First, it is hot-rolled to a predetermined thickness (for example, 30 mm) and width (for example, 2000 mm) in a hot rolling line (not shown) that is separate from the heat treatment equipment 1, and after the temperature reaches room temperature, the scale is removed by an oxide scale removal device. The removed thick steel plate S is charged into the heating furnace 2. Then, the thick steel plate S is heated in the heating furnace 2 to a predetermined temperature.

Next, the thick steel plate S is extracted from the heating furnace 2 and cooled by the water cooling device 3 while being conveyed by a plurality of table rolls 6 installed on the exit side of the heating furnace 2. In this cooling process, the number of zones and the amount of water to be used are calculated and set by the control device 10 according to the plate thickness and target material characteristics.Here, as an example, water is injected from all zones shown in Figure 1. The manufacturing method for this case is shown below.

First, the thick steel plate S injects the cooling water 7 from the cooling water injection nozzles 32 arranged in seven upper and lower pairs. The water density and conveyance speed of the thick steel plate S in the water cooling device are set by the control device 10 and commanded to the cooling water injection nozzle 32 and the table roll 6 so that the target thick steel plate characteristics are obtained. It is.

The thick steel plate S that has undergone this cooling process is subjected to a subsequent process. By passing the thick steel plate S extracted from the heating furnace 2 through the water cooling device, a thick steel plate having desired thick steel plate characteristics (for example, yield ratio of 80% or less) can be manufactured.

Although the embodiments of the present invention have been described above, the present invention is not limited thereto, and various changes and improvements can be made. For example, as shown in FIG. 5, a draining roll 33 may be installed on the outlet side of the water cooling device 3 to drain the cooling water 7 accumulated on the thick steel plate S. By removing the cooling water 7 that has accumulated on the plate, the desired cooling stop temperature and, as a result, the desired characteristics can be obtained more reliably.

In order to obtain good draining properties, the pressing force of the draining roll 33 against the thick steel plate S is preferably 4 tons or more, more preferably 6 tons or more, and still more preferably 8 tons or more. If the pressing force is 20 tons or less, the draining roll 33 will not be deformed and better draining performance will be obtained. Therefore, the pressing force is preferably 20 tons or less.

The mechanism for applying pressing force by the draining roll 33 may be a spring type mechanism such as a spring, or a mechanism capable of applying a constant pressing force such as pneumatic pressure or hydraulic pressure. For the purpose of adjusting the deflection of the draining roll 33, a mechanism that can maintain a constant pressing force is preferable, and a mechanism that has responsiveness that allows the pressing force to be changed in the longitudinal direction of the thick steel plate S is more preferable.

As shown in FIG. 6, a drain purge nozzle 34 may be arranged in place of the drain roll 33, and the cooling water 7 accumulated on the thick steel plate S may be cut off by spraying a drain purge 35. The drain purge 35 may be a liquid or a gas, and a mixed fluid thereof may be injected. In order to keep the temperature deviation of the thick steel plate S smaller, it is preferable to use gas. Furthermore, from the viewpoint of production cost, it is more preferable to use air.

As shown in FIG. 7, a drain roll 33 and a drain purge nozzle 34 may be used together. Further, either or both of a draining roll 33 and a draining purge nozzle 34 may be arranged on the inlet side of the water cooling device 3 to cut off the cooling water 7 leaking from the water cooling device 3. This is because the cooling start temperature of the thick steel plate S can be controlled more accurately.

Furthermore, the draining roll 33 and/or the draining purge nozzle 34 may be arranged not only on the inlet/outlet side of the water cooling device 3 but also on the inlet/outlet side of each cooling zone to separate each cooling zone. This is because when injecting a different amount of water to each cooling zone, the temperature history of the thick steel plate can be ensured by dividing the zones with different amounts of cooling water. In addition, the number of nozzle pairs to be used can be freely set, for example by stopping the injection at the most inlet side of the water cooling device 3 to shorten the cooling time. As a result, the temperature history of the thick steel plate to be cooled can be made more diverse, and appropriate cooling can be performed in accordance with the required characteristics.

As shown in FIG. 8, a large flow rate cooling water injection nozzle 36 (upper large flow rate cooling water injection nozzle 36a and lower side A large flow rate cooling water injection nozzle 36b) may be installed and used to cool the thick steel plate S depending on the target characteristics of the thick steel plate S. Although the number of cooling zones in which the large flow rate cooling water injection nozzles 36 are installed is 3 zones on the inlet side and 3 zones on the outlet side of the water cooling device 3 in FIG. 8, the effect will not be impaired even if the number of zones is other than 3 zones.

Furthermore, as shown in FIG. 9, within the water cooling device 3, the cooling water injection nozzle 32 and the large flow rate cooling water injection nozzle 36 capable of injecting an amount of water outside the range of the invention may be arranged in the same cooling zone. This is because by combining the slow cooling of the present invention with the rapid cooling outside the scope of the present invention, even more diverse temperature histories can be obtained. In FIG. 9, the cooling water injection nozzles 32 and the large flow rate cooling water injection nozzles 36 arranged in the same cooling zone constitute a total of seven zones, but the effect is not impaired even if the zones are other than seven. In addition, a large flow rate cooling water injection nozzle 36 capable of injecting an amount of water outside the scope of the invention may be installed on either or both of the inlet and outlet sides of the water cooling device 3.

Hereinafter, an example in which a thick steel plate was manufactured using the method for manufacturing a thick steel plate according to the present embodiment will be described.

In the equipment shown in Figure 10, thick steel plates (plate thicknesses of 19 mm, 25 mm, and 40 mm x plate width of 3500 mm x plate length of 7 m) at room temperature, from which scale had been previously removed by shot blasting, were heated in a heating furnace to 840°C in a nitrogen atmosphere. . Thereafter, it was cooled with a water cooling device located 2.0 m away from the heating furnace to produce a low yield ratio tempered steel with a yield ratio of 80% or less.

The thick steel plate material to be cooled was heated to 840°C in a small sample heat cycle test conducted in the laboratory, cooled to 450°C at a cooling rate of 6°C/s, and then quickly cooled to room temperature. The structure was ferrite + bainite, and the yield ratio was 75%. The target structure for thick steel plate S was ferrite + bainite, but other tests confirmed that even if part of the plate thickness (for example, near the surface layer) was ferrite + martensite, the properties would not deteriorate significantly. are doing. Therefore, if a thick steel plate is manufactured in an actual heat treatment facility with the same thermal history as the main thermal history, it will be a mixed phase structure of ferrite + bainite, and the yield ratio is expected to be 75%, which is the target structure and low yield ratio. do. Note that a yield ratio of 80% or less was considered acceptable.

The water cooling device 3 was arranged on the outlet side of the heating furnace 2, and seven pairs of upper and lower cooling water injection nozzles 32 were arranged inside the water cooling device 3. Further, a thermometer 4 is disposed on the exit side, so that the surface temperature of the thick steel plate S after cooling can be measured. The thermometer 4 is a scanning thermometer that measures the temperature distribution in the width direction of the thick steel plate S. Among the surface temperatures of the thick steel plate measured over the entire surface of the thick steel plate, the value obtained by subtracting the minimum value from the maximum value was evaluated as the temperature deviation value within the thick steel plate.

In order to obtain a thick steel plate that is uniform over the entire surface, it is necessary to cool the entire surface to a uniform cooling rate and cooling stop temperature. Therefore, we decided to evaluate the uniformity of the characteristics within the thick steel plate using the cooling stop temperature, and those whose temperature deviation value within the thick steel plate was within ±25°C were considered to have passed.

Two types of cooling water injection nozzles 32 were used in combination: a full cone nozzle and a flat spray nozzle. Note that the speed of the cooling water injected from these nozzles was measured in advance in the laboratory, and the results were compared with the results of the operating conditions to confirm whether it was within the scope of the present invention.

When manufacturing thick steel plate S, cooling water is injected so that the average cooling rate in the range from 800°C to 650°C on the surface layer of the thick steel plate is 4°C/s, and the temperature of the surface layer of the thick steel plate at 4 points on the thermometer is 450°C. The number of nozzles 32 used, the water volume density of each zone, and the conveyance speed of the thick steel plate were set. Further, the thick steel plate S discharged from the water cooling device 3 was subjected to rapid cooling, which is a post-process, and was rapidly cooled to room temperature using a well-known technique.

First, in order to understand the relationship between water density and cooling rate, a preliminary experiment was conducted in which a thick steel plate S was heated to a high temperature of 1000°C and then water-cooled to 650°C. The thick steel plate S was heated to 1000° C. in a heating furnace 2 in a nitrogen atmosphere, and then cooled in a water cooling device 3. The surface temperature of the thick steel plate S measured by the thermometer 4 on the outlet side of the water cooling device 3 is 650°C ± 25°C, and the average cooling rate in the range from 800°C to 650°C in the surface layer of the thick steel plate S is 4 to 10°C/s. The number of cooling water injection nozzles 32 used, the water flow density of each zone, and the conveyance speed of the thick steel plate were set as follows.

Further, two types of cooling water injection nozzles 32 were used, a full cone nozzle and a flat spray nozzle, and the results of each were compared. In addition, a one-dimensional heat transfer simulation was carried out based on the surface layer temperature of the thick steel plate S and the heating temperature of the thick steel plate S measured with the thermometer 4 on the outlet side of the water cooling device 3. The average cooling rate was calculated in the range up to ℃.

The relationship between the water volume density and the cooling rate in the cooling zone in the water cooling device 3 is as shown in FIG. 11, and the relationship between the water volume density and the injection speed is as shown in FIG. 12. In other words, the relationship between water density and cooling rate does not change depending on the type of spray. On the other hand, the relationship between water volume density and injection speed was different, and even when spraying at the same water volume density, the full cone nozzle had a lower cooling water injection speed than the flat spray nozzle.

Next, a low yield ratio tempered steel having a yield ratio of 80% or less was manufactured. After the thick steel plate S was heated to 840° C. in a heating furnace 2 in a nitrogen atmosphere, it was cooled in a water cooling device 3. The surface temperature of the thick steel plate S measured by the thermometer 4 on the outlet side of the water cooling device 3 is 450°C ± 25°C, and the average cooling rate of the surface layer of the thick steel plate S in the range from 800°C to 650°C is 6°C/s. , the number of cooling water injection nozzles 32 to be used, the water volume density of each zone, and the transport speed of the thick steel plate were set. Note that the number of cooling water injection nozzles 32 used, the water flow density of each zone, and the transport speed of the thick steel plate were set using the results of the cooling rate measurement experiment described above. Two types of cooling water injection nozzles 32 were used: a full cone nozzle and a flat spray nozzle, and the results were compared.

In addition, a one-dimensional heat transfer simulation was carried out based on the surface layer temperature of the thick steel plate S and the heating temperature of the thick steel plate S, which were measured with the thermometer 4 on the outlet side of the water cooling device 3. Average cooling rates were calculated for a range up to 650°C. The thick steel plate S discharged from the water cooling device 3 was subjected to rapid cooling, which is a post-process, and was rapidly cooled to room temperature using a well-known technique. Thereafter, a small sample was taken from the manufactured thick steel plate S, and a tensile test was conducted to measure the yield ratio and observe the microstructure.

Table 1 shows the manufacturing conditions of the thick steel plate S and the results of the property evaluation test. In the table, F in "Structure" means ferrite, B means bainite, M means martensite, and "R.T." in "Cooling stop temperature" means room temperature.

Examples 1 to 3 are conditions in which water cooling was performed by using a full cone spray nozzle and reducing the jet speed of cooling water. The thick steel plate S could be cooled at the targeted cooling rate and cooling stop temperature, and the yield ratio was within the acceptable range. By reducing the cooling water injection speed, the transition temperature is reduced and the film boiling state can be maintained down to a low temperature, and the average cooling rate in the range from 800°C to 650°C can be maintained down to a low temperature. It is thought that this was due to an accident.

Example 4 is a condition in which scale removal from the surface layer of the thick steel plate was not performed by shot blasting. Although the thick steel plate S was cooled at almost the targeted cooling rate and cooling stop temperature, the temperature deviation in the width direction of the thick steel plate was increased compared to Example 1 in which scale was removed. This is thought to be because the boiling transition temperature of the area where scale had been generated rose, causing partial nucleate boiling and increasing the cooling rate.

Comparative Examples 1 to 3 are conditions in which water cooling was performed using a flat spray nozzle and increasing the cooling water injection speed to a level outside the range of the invention. Although the cooling conditions were set using the results of the cooling rate measurement experiment described above, the cooling stop temperature fell to room temperature. Therefore, the exact cooling rate was unknown. Therefore, a separate test was conducted to identify the cooling rate by shortening the number of zones, and as a result, the cooling rate increased beyond the scope of the present invention. Also, because the cooling rate was high, the yield ratio was outside the acceptable range. This is considered to be because the boiling transition temperature increased and the cooling reached a nucleate boiling state, making it impossible to maintain the average cooling rate in the range from 800°C to 650°C to a low temperature.

Comparative Example 4 is a condition in which water cooling was performed using a flat spray nozzle and slowing down the cooling water injection speed to a level outside the range of the invention. The cooling rate was almost the same as air cooling. In addition, the temperature deviation in the width direction of the thick steel plate failed. This is thought to be because the spray flow rate became low, making it impossible to stably inject cooling water, and water concentrated only directly below the spray.

1 Heat treatment equipment (thick steel plate manufacturing equipment)
2 Heating furnace 3 Water cooling device 4 Thermometer 5 Cooling zone 6 Table roll 7 Cooling water 9 Descaling device 10 Control device 11 Host computer 12 Water cooling condition calculation section 13 Water cooling operation condition output section 32 Cooling water injection nozzle 32a Upper cooling water injection Nozzle 32b Lower cooling water injection nozzle 33 Drain roll 34 Drain purge nozzle 35 Drain purge 36 Large flow rate cooling water injection nozzle 36a Upper large flow rate cooling water injection nozzle 36b Lower large flow rate cooling water injection nozzle S Thick steel plate

Claims (11)

A method for manufacturing a thick steel plate, comprising water cooling the thick steel plate.
Using a water cooling device having a plurality of sets of at least one pair of upper and lower cooling water injection nozzles arranged along the conveyance direction of the thick steel plate,
Water-cooling the thick steel plate by setting the cooling water injection speed of the cooling water injection nozzle to 0.4 m/s or more and 30 m/s or less,
A method for manufacturing a thick steel plate, wherein the surface cooling rate of the thick steel plate in the water cooling device is 0.4°C/s or more and 4.1°C/s or less . The method for manufacturing a thick steel plate according to claim 1, wherein the thick steel plate is descaled before the water cooling. The method for manufacturing a thick steel plate according to claim 1, wherein the thick steel plate is subjected to oxide scale removal treatment and heating before the water cooling. The method for manufacturing a thick steel plate according to claim 1, wherein the temperature of one or both of the upper and lower surfaces of the thick steel plate is measured after the water cooling. The method for manufacturing a thick steel plate according to claim 1 , wherein the surface cooling rate is controlled by controlling the amount of cooling water injected from the cooling water injection nozzle. A cooling stop temperature of the thick steel plate is controlled by controlling the number of the cooling water injection nozzles and a conveyance speed of the thick steel plate, and the cooling stop temperature is set to a surface temperature of the thick steel plate of 350° C. or more. Item 5. The method for producing a thick steel plate according to any one of Items 1 to 5 . A thick steel plate production facility that performs water cooling of thick steel plates,
a water cooling device having a plurality of sets of at least one pair of upper and lower cooling water injection nozzles arranged along the conveyance direction of the thick steel plate;
A control device that controls the injection speed of cooling water injected from the cooling water injection nozzle to 0.4 m/s or more and 30 m/s or less ,
The control device is a manufacturing equipment for thick steel plates, wherein the control device controls a surface cooling rate of the thick steel plates in the water cooling device to 0.4° C./s or more and 4.1° C./s or less . The thick steel plate manufacturing equipment according to claim 7 , further comprising a descaling device on the inlet side of the water cooling device. The thick steel plate manufacturing equipment according to claim 7 , further comprising a thermometer on the exit side of the water cooling device for measuring the temperature of one or both of the upper and lower surfaces of the thick steel plate. The thick steel plate manufacturing equipment according to claim 7 , wherein the control device further controls the amount of cooling water injected from the cooling water injection nozzle. The thick steel plate manufacturing equipment according to any one of claims 7 to 10 , wherein the control device controls the number of the cooling water injection nozzles and the conveyance speed of the thick steel plate.

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