JP6409832B2 - Water quenching apparatus, continuous annealing equipment, and steel plate manufacturing method - Google Patents

Description

  The present invention relates to a water quenching apparatus that can keep the cooling rate of a steel sheet substantially constant, a continuous annealing facility equipped with the water quenching apparatus, and a steel sheet that suppresses variations in mechanical properties by using the water quenching apparatus. The present invention relates to a method for manufacturing a steel sheet capable of manufacturing a steel sheet.

  Annealing is performed in order to soften the steel sheet hardened by cold rolling. Annealing has the process of heating, soaking, and cooling the steel sheet. Depending on the steel type, in the annealing cooling step, water quenching may be performed in which the steel plate is immersed in water contained in the line tank and the steel plate is rapidly cooled.

  Rapid cooling of a steel sheet by water quenching is particularly important in a high-strength cold-rolled steel sheet having a high tensile strength called “HITEN”. By increasing the cooling rate, a hard structure such as martensite and bainite is generated in addition to the ferrite phase. Thereby, the strength of the steel sheet can be increased without adding a special alloy element.

  As the steel sheet is strengthened, it is necessary to suppress the variation of the generated structure in the steel sheet in order to obtain desired mechanical properties. However, there is a problem that it is difficult to control the structure variation because the generated structure varies due to a slight difference in the cooling rate of the steel sheet. Thus, it is known that the mechanical properties of the steel sheet are greatly affected by the cooling conditions during annealing.

  High strength cold-rolled steel sheets for automobiles can be cited as steel sheets that are particularly required to reduce variations in mechanical properties. High-strength cold-rolled steel sheets for automobiles are pressed into a complicated shape during the process of manufacturing automobiles. Due to slight variations in mechanical properties of each steel plate, large dimensional fluctuations occur in the product after press working. In order to perform strict dimensional control, high strength cold-rolled steel sheets for automobiles are required to suppress variations in mechanical properties.

  Conventionally, even when high strength cold-rolled steel sheets are manufactured on the same continuous annealing line using steel types having the same composition, variations in mechanical properties, particularly tensile strength, occur for each steel sheet.

JP 2005-240077 A

  According to the study by the present inventors, it has been found that the variation in the tensile strength of the steel sheet varies depending on the season and the time zone of day and night. When the atmospheric temperature fluctuates depending on the season and time zone, particularly the cooling water temperature in the water-quenched water tank fluctuates. Conventionally, it was common knowledge that the range of temperature fluctuations due to the season of cooling water was only a few dozen degrees and that the slight temperature fluctuation of cooling water had little effect on the mechanical properties of the steel sheet. . As a result of intensive studies by the present inventors, it has been found that the variation in the mechanical properties of the steel sheet can be reduced by reducing the temperature fluctuation due to the season of the cooling water, etc., and the present invention has been conceived.

  Patent Document 1 is cited as a document that discloses a technique for suppressing variation in the cooling rate of a steel sheet during water quenching. In patent document 1, in order to solve the subject of suppressing the variation in the curvature of the steel plate after quenching, the temperature of the water supplied to the water-quenched water tank is set to a substantially constant value, particularly within a range of 50 ° C. ± 5 ° C. (Patent Document 1, paragraphs 0020, 0027, and 0029).

  According to the study by the present inventors, when water quenching is performed at a cooling water temperature within 50 ° C. ± 5 ° C. as in Patent Document 1, the warpage of the steel plate can be effectively suppressed, but a high-strength cold-rolled steel plate is produced in particular. It has been found that there is a problem in that the ductility decreases too much.

  Further, according to the study by the present inventors, it has been found that even if the temperature of the cooling water supplied to the water-quenched water tank is kept constant, fluctuations in the cooling water temperature due to the season in the water-quenched water tank cannot be suppressed. It was done.

  The present invention has been completed in view of the above problems, and by reducing the fluctuation of the cooling water temperature in a water-quenched water tank (hereinafter also referred to as “line tank”) depending on the season and time zone. Manufacturing a steel sheet that suppresses variations in mechanical properties by using a water quenching apparatus that can suppress fluctuations in the cooling rate of the steel sheet, a continuous annealing facility equipped with the water quenching apparatus, and the water quenching apparatus. It is an object of the present invention to provide a method for manufacturing a steel sheet that can be used.

Means of the present invention are as follows.
[1] A water quenching apparatus used for continuous annealing, a line tank that stores cooling water in which a steel plate is immersed, and a part of the cooling water stored in the line tank is extracted and cooled. Cooling water supply equipment for supplying cooling water to the line tank, a cooling water temperature measuring device for measuring the temperature of the cooling water stored in the line tank, and a cooling water temperature measurement result in the line tank by the cooling water temperature measuring device And a control device for controlling the operation of the cooling water supply equipment so that the temperature of the cooling water in the line tank is within T ° C. ± A ° C. A water quenching apparatus characterized by having a temperature of ℃ or less.
[2] A water quenching apparatus used for continuous annealing, a line tank that stores cooling water in which a steel plate is immersed, and a part of the cooling water stored in the line tank is extracted and cooled. Cooling water supply equipment for supplying cooling water to the line tank, a cooling water temperature measuring device for measuring the temperature of the cooling water stored in the line tank, and a cooling water temperature measurement result in the line tank by the cooling water temperature measuring device And a control device for controlling the operation of the cooling water supply facility so that the temperature of the cooling water in the line tank is within T ° C. ± A ° C. A water quenching apparatus characterized in that a difference between a maximum value and a minimum value of tensile strength is a temperature at which the target tensile strength is 12% or less.
[3] The control device is configured to control the cooling water so that the temperature of the cooling water in the line tank is within T ° ± 5 ° C. based on the measurement result of the cooling water in the line tank by the cooling water temperature measuring device. The water quenching apparatus according to [1] or [2], wherein the operation of the supply facility is controlled.
[4] The water quenching apparatus according to any one of [1] to [3], wherein the T ° C is 10 ° C or higher and 50 ° C or lower.
[5] The water quenching apparatus according to [4], wherein the T ° C is set to 30 ° C.
[6] The cooling water supply facility includes a cooling tower for cooling the cooling water, and the control device controls an inflow amount and an outflow amount of the cooling water in the cooling tower. The water quenching apparatus according to any one of [5].
[7] The water quenching apparatus according to any one of [1] to [6], wherein the control device controls an inflow amount and an outflow amount of cooling water in the line tank.
[8] The water quenching apparatus according to any one of [1] to [7], wherein a foreign matter removing device is provided between the cooling water supply facility and the line tank.
[9] A continuous annealing facility comprising the water quenching apparatus according to any one of [1] to [8].
[10] A method for producing a steel sheet using the water quenching apparatus according to any one of [1] to [8], wherein the water temperature in the line tank is within a range of T ° C. ± A ° C. A method for producing a steel sheet, comprising a water quenching step of immersing the steel sheet in the maintained cooling water.
[11] The steel sheet, steel sheet manufacturing method according to [10], which is a high strength cold rolled steel sheet having a 580 M Pa or more tensile strength.

  By using the water quenching apparatus of the present invention, the temperature of the cooling water accommodated in the line tank can be kept within the range of the reference temperature (T ° C) ± A ° C regardless of the temperature change due to the season and time zone. it can. Thereby, the fluctuation | variation of the cooling rate of the steel plate at the time of water quenching can be suppressed, and the dispersion | variation in the mechanical characteristic of the steel plate obtained can be suppressed.

  Furthermore, in the water quenching apparatus of the present invention, the variation in tensile strength can be further reduced by maintaining the temperature of the cooling water accommodated in the line tank within a range of ± 5 ° C. with reference to 30 ° C. At the same time, it is possible to produce a high-strength cold-rolled steel sheet having a desired high tensile strength and preventing a decrease in ductility. Note that 30 ° C. set as the optimum example of the reference temperature (T ° C.) of the cooling water is the facility according to the present invention which is necessary for controlling the temperature range of the cooling water to ± 5 ° C. in common in summer and winter. Is a value that can be manufactured at the lowest cost. That is, when the reference temperature is higher than 30 ° C., a heating device for increasing the temperature of the cooling water circulating in the winter season is further required, whereas when the reference temperature is lower than 30 ° C., the cooling circulating in the summer season is required. A cooling device for lowering the temperature of the water is further required, and the equipment according to the water quenching apparatus of the present invention is expensive. In addition, from the viewpoint of suppressing the cost of the equipment related to the water quenching apparatus, a preferable example of the range of the reference temperature (T ° C) is 10 ° C to 50 ° C, and a further preferable example is 20 ° C to 40 ° C. Is mentioned.

  Moreover, in the other aspect of this invention, it can prevent that a foreign material is carried in in a line tank by using a foreign material removal apparatus. Thereby, generation | occurrence | production of the depression of the product surface by foreign material adhering to the surface of a steel plate at the time of water quenching can be prevented. In particular, in a high-strength cold-rolled steel sheet having high added value, the yield can be remarkably improved by preventing the occurrence of recesses.

FIG. 1 is a flowchart of the water quenching apparatus of the present invention. FIG. 2 is a heat treatment pattern of a steel plate in continuous annealing. FIG. 3 is an explanatory view showing a continuous annealing facility. FIG. 4 is a graph showing the relationship between the cooling water temperature and the tensile strength of the steel sheet in the results of Invention Example 1 and Comparative Example 1. FIG. 5 is a graph showing the relationship between the cooling water temperature and the tensile strength of the steel sheet in the results of Invention Example 2 and Comparative Example 2. FIG. 6 is a graph showing the relationship between the cooling water temperature and the tensile strength of the steel sheet in the results of Invention Example 3 and Comparative Example 3. FIG. 7 is a graph showing the relationship between the cooling water temperature and the tensile strength of the steel sheet in the results of Invention Example 4 and Comparative Example 4.

  First, the continuous annealing facility will be described with reference to FIGS. 2 and 3.

  In FIG. 3, annealing is performed while passing from right to left. The steel plates are (1) pre-tropical, (2) heating zone, (3) soaking zone, (4) precooling zone, (5) water quenching zone, (6) tempering zone, (7) overaging zone, and ( 8) It passes through the cooling zone in order. The relationship between the time and temperature in each of the zones (1) to (8) is shown in FIG. The temperature shown in FIG. 2 is a typical example and is not limited to this example.

  The temperature condition will be described with reference to FIG. First, (1) in the pre-tropical zone, the steel sheet is heated to 300 ° C. Next, (2) the steel sheet is heated to 700 ° C. in the heating zone. Thereafter, (3) in the soaking zone, the temperature of the steel sheet is raised to 800 ° C. and held at 800 ° C. for a certain period of time. (4) In the preliminary cooling zone, the steel sheet is cooled to 650 ° C. using a gas jet or the like. Thereafter, (5) in the water quenching zone, the steel plate is rapidly cooled by being immersed in cooling water accommodated in the line tank 3 shown in FIG. After water quenching, (6) in the tempering zone, it was tempered to around 200 ° C. using an induction heating device, etc. (7) after being held for a certain period of time in the overaging zone, (8) in the cooling zone By cooling, a steel plate near normal temperature is obtained.

  In FIG.2 (b), the relationship between the temperature and time when steel plate temperature cools rapidly in (5) water quenching zone is shown with an enlarged view. In the water quenching zone, the steel plate finally has a temperature comparable to the cooling water temperature stored in the line tank 3. A white circle in the figure shows a conventional example in summer when the temperature of the cooling water in the line tank is about 30 ° C. A square in the figure is a conventional example in winter when the temperature of the cooling water in the line tank is about 15 ° C. Due to the difference in cooling water temperature, the rate at which the steel sheet is cooled to the same temperature as the cooling water temperature differs between summer and winter. In the winter season indicated by the square in the figure, the slope of the straight line is larger and the cooling rate of the steel sheet is higher than in the summer season indicated by the white circle in the figure. Even with a slight difference in cooling water temperature, the production structure differs depending on the cooling rate of the steel sheet, and the mechanical properties of the obtained steel sheet may vary.

  Next, the flow of cooling water used in the water quenching apparatus of the present invention will be described with reference to FIG. In FIG. 1, the flow of cooling water is indicated by a solid line.

  First, the steel plate 1 is immersed in the cooling water accommodated in the line tank 3 so that the steel plate 1 is water-quenched. As described above, the temperature of the steel plate 1 immediately before water quenching has reached about 680 ° C., and the cooling water stored in the line tank 3 can become high temperature by heat exchange. In the present invention, the temperature of the cooling water stored in the line tank 3 can always be maintained within T ° C. ± A ° C. using the control device 14 and the cooling water supply equipment 21. In the example of FIG. 1, the cooling water supply facility 21 includes a circulation tank 5, a cooling tower 6, a cooling tower inlet pump 8A, a cooling tower outlet pump 8B, a cooling tower inlet adjustment valve 9A, and a cooling tower outlet adjustment valve. 9B. T ° ± A ° C. is a temperature range including (T−A) ° C. and (T + A) ° C.

  The cooling water stored in the line tank 3 is supplied from the cooling water supply facility 21 by the tank inlet side pump 7A, and returned to the cooling water supply facility 21 by the tank outlet side pump 7B. In the example of FIG. 1, the cooling water in the line tank 3 is returned to the cooling tower 6 by the tank outlet pump 7B.

  The cooling tower 6 cools water. The cooled water is extracted from the cooling tower by the cooling tower outlet pump 8B and temporarily stored in the circulation tank 5. A part of the cooling water stored in the circulation tank 5 is sent to the line tank 3 by the tank inlet side pump 7A and partly sent to the cooling tower 6 by the cooling tower inlet side pump 8A. Is done. A cooling tower inlet adjustment valve 9A for adjusting the amount of inflow into the cooling tower 6 is provided in the vicinity of the cooling tower inlet pump 8A, and a cooling tower 6 from the cooling tower 6 is provided in the vicinity of the cooling tower outlet pump 8B. A cooling tower exit side adjustment valve 9B for adjusting the outflow amount is provided.

  A conventionally known apparatus can be used as the cooling tower 6. As a typical example, an open type cooling tower in which the cooling water supplied into the cooling tower is brought into contact with the atmosphere and the cooling water is cooled by heat of vaporization of water can be used. In general, the cooling tower 6 is a device that can cool water from an atmospheric temperature to a temperature that is lower than the heat of vaporization of water, and the cooling capacity depends on the atmospheric temperature. Therefore, the temperature of the cooling water cooled in the cooling tower 6 tends to be high in summer and low in winter.

  In FIG. 1, an arrow from the circulation tank 5 to the line tank 3 is shown as a line for supplying cooling water from the cooling water supply facility 21 to the line tank 3. The water stored in the circulation tank 5 is supplied to the line tank 3 by the tank inlet pump 7A. It is preferable that a foreign matter removing device 4 represented by a strainer or the like is provided between the circulation tank 5 and the line tank 3. By passing cooling water through the foreign matter removing device 4, foreign matters can be prevented from being brought into the line tank 3. Thereby, it can prevent that a foreign material adheres to the steel plate surface at the time of water quenching, and the dent wrinkle derived from a foreign material generate | occur | produces, and can obtain the steel plate with the favorable surface property. The types of foreign matter that can be removed by the foreign matter removing device 4 include sludge such as iron scraps brought from the surface of the steel plate 1 during water quenching in the line tank 3, insects and sand sucked together with the atmosphere in the cooling tower 6. Examples include flying objects.

  Next, the control mechanism of the cooling water temperature in the line tank 3 will be described with reference to FIG. In FIG. 1, the control flow by the control device 14 is indicated by a dotted line.

  The line tank 3 is provided with a cooling water temperature measuring device 13. The cooling water temperature measuring device 13 measures the temperature of the cooling water stored in the line tank 3. The cooling water temperature measuring device 13 is connected to the control device 14. The control device 14 adjusts the operating conditions of the cooling water supply equipment 21 so that the cooling water temperature in the line tank is always maintained within the range of T ° C. ± A ° C.

  As an example of the operating conditions controlled by the control device 14, there are an inflow amount and an outflow amount of cooling water in the cooling tower 6. More specifically, the amount of cooling water flowing into the cooling tower 6 can be adjusted by controlling the cooling tower inlet pump 8A and the cooling tower inlet adjustment valve 9A. Similarly, the outflow amount of the cooling water from the cooling tower 6 can be adjusted by controlling the cooling tower outlet pump 8B and the cooling tower outlet adjustment valve 9B. When the cooling water temperature in the line tank 3 is likely to exceed (T + A) ° C., for example, under conditions where the cooling water temperature is likely to be high, such as in summer, the amount of inflow and outflow to the cooling tower 6 is increased and the cooling tower 6 performs processing. Increasing the amount of cooling water produced. In addition, when the cooling water temperature in the line tank 3 is likely to be lower than (TA) ° C., for example, under conditions where the cooling water temperature tends to be low, such as in winter, the amount of inflow and outflow to the cooling tower 6 is reduced and cooling is performed. The amount of cooling water treated in the tower 6 is reduced.

  Another example of the operating conditions controlled by the control device 14 is the amount of cooling water inflow and outflow in the line tank 3. More specifically, the operation of the tank inlet side pump 7A and the tank outlet side pump 7B is controlled. When the cooling water temperature in the line tank 3 is likely to exceed (T + A) ° C., it is possible to increase the flow rate of the tank inlet side pump 7A and the tank outlet side pump 7B to increase the cooling water exchange speed inside the line tank. Done. On the other hand, when the cooling water temperature in the line tank 3 is likely to be lower than (TA) ° C., the flow rate of the tank inlet side pump 7A and the tank outlet side pump 7B is decreased to replace the cooling water inside the line tank. Decreasing the speed is done.

  In addition, the apparatus controlled by the control apparatus 14 is not limited to the said example. For example, although not shown, the temperature of the cooling water in the line tank 3 can also be adjusted by adjusting the air volume of the cooling tower 6 by the control device 14.

  A ° C. can be more than 0 ° C. and 10 ° C. or less. By setting it as such temperature conditions, the dispersion | variation in the mechanical characteristic in the steel plate after water quenching can be prevented. From the viewpoint of more reliably suppressing variations in mechanical properties, it is preferable to set A ° C to 5 ° C.

  A ° C. may be a temperature at which the difference between the maximum value and the minimum value of the tensile strength of the obtained steel sheet is 12% or less of the target tensile strength. More specifically, the tensile strength (minimum value) of a steel plate obtained by water quenching at a water temperature of T ° C. + A ° C. and the tensile strength of the steel plate obtained by water quenching at a water temperature of T ° C.-A ° C. A ° C. is determined so that the difference from the strength (maximum value) is 12% or less of the target tensile strength of the target steel sheet. By determining A ° C. in this way, it is possible to reliably prevent variations in the mechanical properties (specifically, tensile strength) of the steel sheet. In particular, since a steel sheet having a target tensile strength error (deviation) exceeding ± 6% may affect the shipment of products, such a temperature setting can suppress a decrease in yield. The maximum value and the minimum value of the tensile strength of the steel sheet can be determined by conducting a water quenching test on the steel sheet to be passed through and measuring the actual tensile strength obtained after the test. When a plurality of tests are performed, the arithmetic average value of the obtained tensile strength can be used as the maximum value / minimum value of the tensile strength.

  The target tensile strength of the steel sheet is not particularly limited. However, since water quenching is often performed with a high-strength cold-rolled steel sheet (HITEN), the target tensile strength is preferably 580 MPa or more.

  The T ° C (reference temperature) is preferably 10 ° C or higher and 50 ° C or lower. If the reference temperature exceeds 50 ° C., the effect of quenching the steel sheet cannot be obtained sufficiently, and if it falls below 10 ° C., a large cost is required to maintain the cooling water temperature. Among these, it is particularly preferable to set the reference temperature to 30 ° C. from the viewpoint of suppressing the cooling cost in summer.

  More preferably, the cooling water temperature inside the line tank 3 is adjusted so that the reference temperature (T ° C.) is 30 ° C. and is 25 ° C. or more and 35 ° C. or less. When the cooling water temperature is less than 25 ° C., the cooling rate of the steel sheet becomes too fast, the martensite fraction becomes excessive, the tensile strength becomes excessive, and the ductility of the steel sheet decreases. On the other hand, if the cooling water temperature is higher than 35 ° C., the cooling rate of the steel sheet becomes too slow, the martensite fraction becomes too low, and a steel sheet having sufficient tensile strength cannot be manufactured.

  Further, in the present invention, the mechanical properties of the steel sheet capable of suppressing the variation are not limited to the tensile strength, and other mechanical properties such as ductility are suppressed due to variations in the cooling rate during water quenching. be able to.

  When a cooling tower, particularly an open type cooling tower is used, the cooling water is difficult to cool in summer. In particular, maintaining the cooling water temperature in the line tank below 25 ° C. during the daytime in summer is very expensive. The cooling water temperature is preferably within the range of 30 ° C. ± 5 ° C. from the viewpoint of suppressing the cooling cost in summer.

  The steel plate produced in the present invention is preferably a high-strength cold-rolled steel plate (high tensile), and specifically, a steel plate having a tensile strength of 580 MPa or more. Although the upper limit of the tensile strength is not particularly limited, an example is 1600 MPa or less. As specific examples of the composition of the high-strength cold-rolled steel sheet, by mass%, C is 0.04% or more and 0.220% or less, Si is 0.01% or more and 2.00% or less, and Mn is 0.80% or more. 2.80% or less, P is 0.001% or more and 0.090% or less, S is 0.0001% or more and 0.0050% or less, sol. Al is 0.005% or more and 0.065% or less, and if necessary, at least one of Cr, Mo, Nb, V, Ni, Cu, and Ti is 0.5% or less, and if necessary, B , Sb is 0.01% or less, and other Fe and unavoidable impurities.

(Invention Example 1), (Comparative Example 1)
Steel plate with target tensile strength of 590 MPa (Chemical component: C; 0.093%, Si; 0.20%, Mn; 0.85%, P; 0.07%, S; 0.0020%, sol.Al 0.050%, balance iron and unavoidable impurities) were passed through the continuous annealing furnace shown in FIG. The temperature and time in the continuous annealing furnace were the conditions shown in FIG. Moreover, it experimented by changing as the cooling water temperature in the line tank 3 in the range from 15 degreeC-62 degreeC. An example in which the cooling water temperature in the line tank 3 is within an arbitrary reference temperature (T ° C.) ± 5 ° C. is referred to as Invention Example 1, and the cooling water temperature is 15 ° C. to 62 ° C. (38.5 ° C. ± 23.5 ° C.) Was designated as Comparative Example 1.

  The results are shown in FIG. For example, when the reference temperature is set to 30 ° C. and the cooling water temperature in the line tank is set to a range of 25 ° C. to 35 ° C., the tensile strength is in the range of 600 MPa to 640 MPa, and variation is suppressed. On the other hand, when the cooling water temperature in the line tank was viewed throughout the range of 15 ° C. to 62 ° C., the tensile strength was a minimum of 520 MPa and a maximum of 670 MPa, and the variation in tensile strength was large.

  Moreover, in the example whose cooling water temperature is less than 25 degreeC, tensile strength is excessive and has caused the fall of ductility, and in the example whose cooling water temperature is higher than 35 degreeC, sufficient tensile strength was not obtained. From the above, it was shown that by setting the reference temperature to 30 ° C. and maintaining the cooling water temperature within 30 ° C. ± 5 ° C., a steel sheet having sufficient tensile strength and suppressing variations in tensile strength can be obtained.

(Invention Example 2), (Comparative Example 2)
Steel sheet having a target tensile strength of 980 MPa (chemical component: C; 0.125%, Si; 1.40%, Mn; 1.90%, P; 0.010%, S; 0.001%, sol.Al) 0.030%, the remaining iron and unavoidable impurities) were tested in the same manner as Example 1 and Comparative Example 1. Experiments were performed while varying the cooling water temperature in the line tank 3 from 12 ° C to 36 ° C.

  The results of Invention Example 2 and Comparative Example 2 are shown in FIG. The “WQ water temperature” on the horizontal axis in FIGS. 5 to 7 means the cooling water temperature in the line tank 3. When the cooling water temperature in the line tank is within 30 ° C. ± 5 ° C. as shown by the arrow of “Example of the present invention” in FIG. 5, the difference between the maximum value and the minimum value of the tensile strength in the obtained steel sheet. Was 111 MPa. On the other hand, when the cooling water temperature in the line tank was in the entire range of 12 ° C. to 36 ° C. (24 ° C. ± 12 ° C.), the difference between the maximum value and the minimum value of the tensile strength was 160 MPa. Thus, by setting the reference temperature to 30 ° C. and the cooling water temperature within the range of 30 ° C. ± 5 ° C., variation in tensile strength could be suppressed.

(Invention Example 3), (Comparative Example 3)
Steel plate having a target tensile strength of 780 MPa (chemical component: C; 0.065%, Si; 1.00%, Mn; 2.20%, P; 0.008%, S; 0.002%, sol.Al) 0.035%, the remaining iron and unavoidable impurities) were tested in the same manner as Example 1 and Comparative Example 1. Experiments were performed while varying the cooling water temperature in the line tank 3 from 12 ° C to 41 ° C.

  The results of Invention Example 3 and Comparative Example 3 are shown in FIG. When the cooling water temperature in the line tank is within 28 ° C. ± 7 ° C. as shown by the arrow of “Example of the present invention” in FIG. 6, the difference between the maximum value and the minimum value of the tensile strength in the obtained steel sheet. Was 92 MPa. On the other hand, when comparing only the data satisfying the 780 MPa standard for the entire range of the cooling water temperature in the line tank of 12 ° C. to 41 ° C. (26.5 ° C. ± 14.5 ° C.), the maximum value and the minimum value of the tensile strength are The difference was 143 MPa. Thus, by setting the reference temperature to 28 ° C. and the cooling water temperature within the range of 28 ° C. ± 7 ° C., it was possible to suppress variations in tensile strength.

(Invention Example 4), (Comparative Example 4)
Steel plate having a target tensile strength of 1180 MPa (chemical component: C; 0.135%, Si; 1.40%, Mn; 2.10%, P; 0.010%, S; 0.001%, sol.Al 0.030%, the remaining iron and unavoidable impurities) were tested in the same manner as Example 1 and Comparative Example 1. Experiments were performed while varying the cooling water temperature in the line tank 3 from 12 ° C to 43 ° C.

  The results of Invention Example 4 and Comparative Example 4 are shown in FIG. When the cooling water temperature in the line tank is within 31 ° C. ± 4 ° C. as indicated by the arrow of “Example of the present invention” in FIG. 7, the difference between the maximum value and the minimum value of the tensile strength in the obtained steel sheet. Was 129 MPa. On the other hand, when comparing only the data satisfying the 1180 MPa standard for the entire range where the cooling water temperature in the line tank is 12 ° C. to 43 ° C. (27.5 ° C. ± 15.5 ° C.), the maximum value and the minimum value of the tensile strength are The difference was 171 MPa. Thus, the variation in tensile strength could be suppressed by setting the reference temperature to 31 ° C. and the cooling water temperature within the range of 31 ° C. ± 4 ° C.

DESCRIPTION OF SYMBOLS 1 Steel plate 3 Line tank 4 Foreign material removal apparatus 5 Circulation tank 6 Cooling tower 7A Tank entrance side pump 7B Tank exit side pump 8A Cooling tower entrance side pump 8B Cooling tower exit side pump 9A Cooling tower entrance side adjustment valve 9B Cooling tower exit side adjustment Valve 11 Water quenching device 13 Cooling water temperature measuring device 14 Control device 21 Cooling water supply equipment

Claims (9)

A water quenching apparatus used for continuous annealing for producing a steel sheet having a tensile strength of 580 MPa or more ,
A line tank containing cooling water into which the steel plate is immersed,
A cooling water supply facility that extracts and cools a part of the cooling water stored in the line tank, and supplies the cooled cooling water to the line tank;
A cooling water temperature measuring device for measuring the temperature of the cooling water stored in the line tank;
Control for controlling the operation of the cooling water supply equipment so that the temperature of the cooling water in the line tank is within T ° C. ± A ° C. based on the measurement result of the cooling water temperature in the line tank by the cooling water temperature measuring device. An apparatus,
The control device is configured to control the cooling water supply facility so that the cooling water temperature in the line tank is within T ° ± 7 ° C. based on the cooling water temperature measurement result in the line tank by the cooling water temperature measuring device. Control operation,
The water quenching apparatus, wherein the T ° C is set to 28 ° C to 31 ° C. 2. The water quenching apparatus according to claim 1, wherein the A ° C. is a temperature at which the difference between the maximum value and the minimum value of the tensile strength of the steel sheet to be obtained is 12% or less of the target tensile strength.   The control device is configured to control the cooling water supply equipment so that the cooling water temperature in the line tank is within T ° ± 5 ° C. based on the cooling water temperature measurement result in the line tank by the cooling water temperature measuring device. The water quenching apparatus according to claim 1 or 2, wherein operation is controlled. The water quenching apparatus according to any one of claims 1 to 3 , wherein the T ° C is set to 30 ° C. The cooling water supply facility has a cooling tower for cooling the cooling water,
The water quenching apparatus according to any one of claims 1 to 4 , wherein the control device controls an inflow amount and an outflow amount of cooling water in the cooling tower. The said control apparatus controls the inflow amount and outflow amount of the cooling water in a line tank, The water quenching apparatus as described in any one of Claim 1-5 characterized by the above-mentioned. The water quenching apparatus according to any one of claims 1 to 6 , wherein a foreign matter removing device is provided between the cooling water supply facility and the line tank. A continuous annealing facility comprising the water quenching apparatus according to any one of claims 1 to 7 . A method for producing a steel sheet having a tensile strength of 580 MPa or more performed using the water quenching apparatus according to any one of claims 1 to 7 ,
A steel plate characterized by comprising a water quenching step of immersing the steel plate in cooling water whose water temperature in the line tank is maintained within a range of T ° C ± 7 ° C , wherein the T ° C is 28 ° C or higher and 31 ° C or lower. Manufacturing method.

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