JPH0890046A - Cooling method of hot steel plate - Google Patents

Abstract

PURPOSE: To prevent generation of poor flatness of a steel plate by controlling injection speed in response to the temp. distribution in the width direction of the steel plate in executing hot straightening after the hot rolled plate is rapidly cooled while transferring in the longitudinal direction. CONSTITUTION: The cooling device used is provided with nozzle headers 22 arranged with nozzles 21 in the prescribed interval wa in the longitudinal direction and in the fixed interval wb of a steel plate 1 and is capable of controlling a cooling water quantity from the nozzle 21 of nozzle header line 22. The temp. distribution on the surface of steel plate 1 at the inlet or outlet of hot straightening machine following to the cooling device is measured to obtain average temp. distribution in the width direction, the residual stress distribution and critical buckling stress after the virtual steel plate having this temp. distribution in the longitudinal direction in row is air cooled to normal temp. are operated and estimated. Successively, based on the operated value, presence/absence of generation of buckling deformation at normal temp. is discriminated, in the case of presence of buckling deformation, the correcting instruction value of cooling water quantity is obtained so that the residual stress is less than critical buckling stress, water quantity control to each nozzle header line 22 is executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to heat treatment for preventing the occurrence of flat defects due to cooling when cooling high temperature steel sheets such as direct quenching and accelerated cooling after hot rolling of steel sheets. The present invention relates to a method for cooling a steel sheet.

[0002]

2. Description of the Related Art In recent years, in a steel plate manufacturing process such as a thick plate, a technique for obtaining a high strength and high toughness steel plate by water cooling a hot steel plate after controlled rolling (hereinafter referred to as accelerated cooling) has been widely used. Is coming. In other words, accelerated cooling is achieved by combining controlled rolling and accelerated cooling to achieve higher strength and toughness of steel sheets, which has been achieved by increasing the conventional additive element components, etc. It is possible not only to significantly reduce the manufacturing cost, but also to manufacture a steel sheet having excellent weldability.

As described above, accelerated cooling can produce a steel sheet of excellent quality at a low cost, but on the other hand, as the needs for higher quality increase recently, there are some problems. This is occurring, and an important problem among them is the occurrence of poor flatness of the accelerated cooled steel sheet. That is, in accelerated cooling, it is general to inject cooling water from a cooling nozzle onto the surface of a steel sheet having a high temperature of 200 ° C. to 900 ° C. to forcibly cool the steel sheet. At this time, convection occurs on the surface of the steel sheet. A boiling heat transfer phenomenon occurs. In accelerated cooling, due to this convective boiling heat transfer phenomenon, a high cooling rate of several tens to several hundred times higher than natural cooling can be achieved, a steel sheet having a finer crystal structure is obtained, and the above-mentioned high strength and high toughness are obtained. It is possible to manufacture a steel plate having

On the other hand, however, in this convective boiling heat transfer phenomenon, the heat transfer coefficient tends to increase as the surface temperature of the steel plate becomes lower, and therefore the phenomenon itself is very unstable. For example, if there is a slight temperature unevenness on the steel plate surface at the start of cooling, no matter how evenly the water quantity density of the cooling water, which is one of the factors affecting the heat transfer coefficient of the steel plate surface, is controlled over the entire steel plate surface, Since the cooling is further promoted in the low temperature region, a large temperature unevenness will occur after the cooling is completed. The occurrence of such temperature unevenness not only finally causes variations in the mechanical property values of the steel sheet, but also when the temperature unevenness exceeds a certain temperature difference, a seismic wave or medium elongation occurs until it cools to room temperature. However, there is a problem that such a defective shape is generated.

As a method of preventing the defective shape of the steel sheet in the above accelerated cooling, in cooling the steel sheet, the temperature distribution in the sheet width direction is measured before the start of forced cooling, and a uniform temperature distribution in the width direction is obtained after cooling. A method of covering the steel plate width end with a shielding gutter, preventing the cooling water from directly contacting the steel plate width end, and preventing the temperature drop of the steel plate width end based on the result of the calculation as JP-A-58-32511), which is installed in a hot steel plate conveying line and has nozzle headers extending in the plate width direction above and below the hot steel plate, respectively, and a large number of cooling water jets in the longitudinal direction of the nozzle header. When forcibly cooling the hot steel sheet immediately after rolling using a cooling device divided into a plurality of zones, which is provided so that the nozzles face the steel sheet surface, the temperature of the steel sheet is set after the rolling and before it is charged into the cooling device. Actual measurement Then, by predicting the steel sheet temperature until the end of cooling from moment to moment in each of the thickness direction, the length direction and the width direction of the steel sheet, the cooling conditions of the predetermined cooling start temperature, stop temperature and cooling rate are determined. Required cooling zone length, upper and lower surface water amount density, strip speed in front of and in the cooling device, strip speed pattern in the cooling device, width direction estimation so that uniform cooling can be achieved. Obtaining the density respectively, after cooling the hot plate through the cooling device based on the cooling schedule obtained,
A method of actually measuring the temperature of the steel plate after cooling, obtaining an error from the predicted value, and performing inter-plate learning for predicting the steel plate temperature (JP-A-60-879).
No. 14), the temperature at each temperature measuring point and the temperature between the temperature measuring points are detected by detecting the temperature before starting water cooling, during water cooling and after water cooling at least in the central portion and the side end portions in the vertical direction and the width direction of the steel sheet. The difference is calculated, the deformation amount in the normal temperature region of the steel sheet is predicted and calculated based on a predetermined relational expression corresponding to the temperature at each temperature measurement point and the temperature difference between the temperature measurement points, and the predicted value is the target value. A method (Japanese Patent Publication No. 63-47775) of controlling the supply amount of cooling water to a plurality of nozzles so as to be within an allowable range has been proposed.

As a method of estimating the amount of deformation of a steel sheet, when a steel sheet having a temperature distribution is cut into a predetermined shape,
A method of estimating the amount of deformation of a partial material obtained by cutting, measuring the plate surface temperature distribution of a steel plate two-dimensionally,
There has been proposed a method (Japanese Patent Laid-Open No. 62-236617) in which a deformation amount is estimated and calculated using the measured plate surface temperature distribution data and a predetermined estimation formula that incorporates residual stress.

[0007]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The method disclosed in Japanese Patent No. 511 aims to prevent the occurrence of corrugated deformation of the steel sheet by preventing the steel sheet width end from being overcooled, and uneven surface properties of the steel sheet, fluctuation of cooling, etc. As a result, supercooling of a portion other than the width end portion of the steel sheet may occur, and it is not possible to prevent wavy deformation caused by this supercooling. Further, the prevention of the supercooling of the width end is performed based on the temperature measurement result before cooling, and it is very difficult to realize the temperature uniformity of the steel sheet. Further, the method disclosed in JP-A-60-87914 prevents deformation of the steel sheet by measuring the steel sheet temperature before and after cooling and controlling the cooling so as to make the temperature of the steel sheet uniform at the end of cooling. Is the way. However, the cooling characteristics of the steel sheet change subtly due to the surface properties of the steel sheet, fluctuations of cooling water, etc., so even if the steel sheet temperature before and after cooling of a certain cooled steel sheet is measured to find a water quantity density capable of uniform cooling, Unless the above-mentioned factors affecting the cooling characteristics can be completely reproduced, it is impossible to achieve completely uniform cooling in other steel sheets. The deformation of the steel sheet can be prevented if the temperature of the steel sheet can be made completely uniform, but it is practically impossible to make the temperature of the steel sheet completely uniform. Therefore, in order to prevent deformation of the steel sheet, it is necessary to clarify the allowable range of temperature unevenness in the range where deformation can be prevented. Further, the method disclosed in Japanese Examined Patent Publication No. 63-47775 predicts the amount of deformation of the steel sheet in the normal temperature range based on a predetermined relational expression corresponding to the temperature at each temperature measuring point and the temperature difference between the temperature measuring points. However, there is a drawback that the deformation amount cannot be accurately predicted only by the temperature at each temperature measuring point and the temperature difference between the temperature measuring points.

Furthermore, the method disclosed in JP-A-62-236617 measures the temperature distribution of a steel sheet and predicts the residual stress of the steel sheet at room temperature. Although it can be applied to predict the bending deformation of strips, it is not possible to predict the waving deformation (ear wave, medium elongation) in accelerated cooling of steel sheet, and the amount of ripping deformation and the amount of waving deformation do not correspond to each other. Absent. As described above, the conventional technique may be able to partially prevent the flatness failure of the steel sheet, but cannot reliably prevent the flatness failure, so that the quality problem remains and the occurrence of the flatness failure at room temperature occurs. In order to confirm the presence or absence, the steel sheet after cooling still has the inconvenience in the process such as not being able to be immediately transferred to the refining process.

An object of the present invention is to solve the above-mentioned problems of the prior art, predict the residual stress distribution until the temperature reaches normal temperature immediately after accelerated cooling in the accelerated cooling of hot steel sheets, and calculate the residual stress distribution of the plate material from the residual stress distribution. An object of the present invention is to provide a method for cooling a hot steel sheet that can obtain a steel sheet having a good flat shape by predicting the flat shape.

[0010]

Means for Solving the Problems The inventors of the present invention have thought that the corrugated deformation at room temperature is a buckling caused by an internal stress generated by a non-uniformly cooled steel sheet until it reaches room temperature. Repeated trial studies. FIG. 3 (a) is the temperature distribution in the width direction of the steel sheet measured in the hot straightening part that performs hot straightening after accelerated cooling, and FIG. 3 (b) naturally cools the steel sheet after hot straightening to room temperature. It is an example of a distribution of residual stress in the width direction of the steel sheet measured in this case. The residual stress distribution of the steel plate and the temperature distribution in the hot straightening part correspond well as shown in FIGS. 3 (a) and 3 (b), and the stress inside the steel plate is temporarily released by the hot straightening by the hot straightening machine. Then, it can be seen that the residual stress is generated due to the temperature distribution remaining in the steel sheet at the time of finishing the hot straightening. In addition, as shown in FIG.
The non-uniform temperature distribution caused by rapid cooling is not a simple case where only the width end is relatively cooler than the other parts, and the relatively cool supercooled region is in the width direction of the steel sheet. Distribution is more common. Further, in a steel sheet having such a temperature distribution, the residual stress distribution generated at normal temperature also has a complicated stress distribution in which tensile and compressive stress regions are distributed in the width direction of the steel sheet. The limit of occurrence of buckling under a uniform compressive stress or a distributed stress such as a combination of a uniform compressive stress and a bending stress is conventionally known, but has a complicated distribution as shown in FIG. 3B. The occurrence of buckling under internal stress has not been clarified yet. Therefore, the present inventor theoretically analyzes the limit of buckling occurrence by using a three-dimensional finite element method for the internal stress of various distribution forms in which the stress distribution in the width direction of the steel sheet repeatedly includes tensile and compression regions. Then, the following findings were obtained.

1) Buckling of a steel sheet occurs when the residual stress of compression exceeds a certain limit (critical buckling stress), and this critical buckling stress changes depending on the stress distribution shape. For example,
FIG. 4 shows that there is only one supercooling zone in the width direction of a steel plate having a plate thickness of 24 mm and a plate width of 3.28 m, as shown in FIG. 4 (a), and compressed at room temperature as shown in FIG. 4 (b). An example in which the residual stress is generated at only one position in the width direction will be shown.
In this case, as shown in FIG. 5, the critical buckling stress changes depending on the position of the compressive residual stress, and the critical buckling stress becomes maximum when the compressive residual stress is located in the width center part. Therefore, the buckling limit temperature deviation (ΔTmax) _CR that causes the flatness of the steel sheet also changes depending on the widthwise position of the compressive residual stress region, that is, the widthwise position of the supercooling region. When the width of the supercooled region and the temperature deviation ΔT are equal, the magnitude of the compressive residual stress generated is the same regardless of the position of the supercooled region in the width direction of the steel sheet. However, since the critical buckling stress changes depending on the position in the width direction of the compressive residual stress region, it does not buckle when the supercooled region is at the center of the width, but the width of the supercooled region and the temperature deviation ΔT are the same. Even if there is, a phenomenon such as buckling occurs when the supercooled region is at the width end.

2) When there are a plurality of compressive residual stress regions, as shown in FIGS. 6 and 7, when the temperature distribution changes, the stress distribution changes and the critical buckling stress distribution also changes. The maximum temperature difference (ΔTma) in the widthwise temperature distribution (buckling critical temperature distribution) that causes the critical buckling stress distribution
x) The _CR value also greatly varies depending on the temperature distribution. In FIGS. 6 and 7, the temperature difference ΔT in the width direction is (ΔTma
x) If it is larger than _CR , it means that the compressive residual stress becomes higher than the critical buckling stress and buckling occurs. As shown in FIG. 6, the maximum temperature deviation (ΔTmax) _CR of the buckling limit is
It increases if the temperature distribution is width symmetric. Further, in the case of the width symmetric temperature distribution, as shown in FIG. 7, the temperature becomes maximum when the relative low temperature parts reach 1/4 of the plate width. Thus, the value of the maximum buckling limit temperature deviation (ΔTmax) _CR in the width direction is
Depending on the temperature distribution, it is three times or more different.

As is clear from the above, the presence or absence of buckling due to the internal stress of the steel sheet should be discussed not only by the magnitude of the deviation of the temperature unevenness but by the distribution of the compressive residual stress. Moreover, it is not enough to discuss the magnitude of the compressive residual stress, and it is clarified that it can be determined by the overall form of the compressive residual stress region such as the width, position, and number of the compressive residual stress. Reached

That is, the first invention of the present application is a method of cooling a hot steel sheet in a step of accelerating cooling while transporting the hot steel sheet in the longitudinal direction and then performing hot straightening, in which nozzle headers are arranged at predetermined intervals in the longitudinal direction of the steel sheet. Nozzle header rows provided at predetermined intervals in the steel plate width direction, using a cooling device having a flow rate adjusting mechanism capable of controlling the amount of cooling water from the injection nozzles of each nozzle header row, of the hot straightener following the cooling device. The temperature distribution of the entire upper surface of the steel sheet is measured at the inlet or the outlet, and the average widthwise temperature distribution of the entire upper surface of the steel sheet is compared with a preset set value to control the amount of cooling water to each nozzle header row of the cooling device. The method for cooling a hot steel plate is characterized by

The second invention of the present application is a method for cooling a hot steel plate in a step of accelerating cooling while transporting the hot steel plate in the longitudinal direction and then performing hot straightening, in which nozzle nozzles are provided at predetermined intervals in the longitudinal direction of the steel plate. A hot straightening machine is provided which is provided with nozzle header rows arranged at predetermined intervals in the width direction of the steel plate, and which uses a cooling device having a flow rate adjusting mechanism capable of controlling the amount of cooling water from the injection nozzles of each nozzle header row. The average width-direction temperature is measured by measuring the temperature distribution over the entire upper surface of the steel sheet at the inlet or outlet and averaging the width-direction temperature distribution at each position in the length direction of the steel sheet based on the temperature distribution over the entire upper surface of the steel sheet. Obtaining the distribution, virtual steel sheet having this average width direction temperature distribution continuously in the length direction is calculated and predicted residual stress distribution and critical buckling stress after being allowed to cool to room temperature,
Based on the predicted residual stress and critical buckling stress, it is determined whether or not buckling deformation occurs at room temperature, and if there is buckling deformation, the residual stress is less than the critical buckling stress,
Inverse calculation of the steel plate in-plane temperature distribution at the inlet or outlet of the hot straightening machine, depending on the difference between the measured steel plate in-plane temperature distribution at the inlet or outlet of the hot straightening machine, in the width direction of the cooling device A method for cooling a hot steel sheet, comprising: calculating a correction instruction value for the amount of cooling water in each nozzle header row and controlling the amount of cooling water to each nozzle header row of the cooling device.

[0016]

In the first invention of the present application, the nozzle header rows in which nozzle headers are arranged at predetermined intervals in the longitudinal direction of the steel sheet are provided at predetermined intervals in the steel sheet width direction, and the amount of cooling water from the injection nozzles of each nozzle header row is controlled. Using a cooling device having a possible flow rate adjusting mechanism, the temperature distribution of the entire upper surface of the steel sheet is measured at the inlet or outlet of the hot straightening machine following the cooling device, and the average width direction temperature distribution of the entire upper surface of the steel sheet is measured. By comparing the amount of cooling water to each nozzle header row of the cooling device by comparing with a preset set value, it becomes possible to control the amount of cooling water in the steel plate width direction, and the temperature in the length direction and width direction of the steel plate. It is possible to control the distribution substantially uniformly, prevent seismic waves, inward extension, etc. due to internal stress of the steel sheet, and manufacture a steel sheet having a good flat shape.

In the second invention of the present application, nozzle header rows in which nozzle headers are arranged at predetermined intervals in the longitudinal direction of the steel sheet are provided at predetermined intervals in the steel sheet width direction, and the amount of cooling water from the injection nozzles of each nozzle header row is set. Using a cooling device having a controllable flow rate adjusting mechanism, the temperature distribution of the entire upper surface of the steel sheet is measured at the inlet or outlet of the hot straightening machine following the cooling device, and based on the temperature distribution of the entire upper surface of the steel sheet. The average width-direction temperature distribution is obtained by averaging the width-direction temperature distribution at each position in the length direction of the steel plate, and the virtual steel plate having this average width-direction temperature distribution continuously in the length direction is released to room temperature. The residual stress distribution after cooling and the critical buckling stress are calculated and predicted. Based on the predicted residual stress and critical buckling stress, it is determined whether or not buckling deformation occurs at room temperature. In the case of, so that the residual stress is less than the critical buckling stress, the temperature distribution in the steel sheet plane at the inlet or outlet of the hot straightening machine is calculated backward, and the steel sheet surface at the inlet or outlet of the hot straightening machine is measured. Depending on the difference with the internal temperature distribution, the correction instruction value of the cooling water amount of each nozzle header row in the width direction of the cooling device is calculated, and by controlling the cooling water amount to each nozzle header row of the cooling device,
It is possible to control the temperature distribution in the length direction and the width direction of the steel plate to a temperature distribution where buckling deformation does not occur at room temperature,
Prevents seismic waves, inward extension, etc. due to internal stress of the steel plate,
A steel plate having a good flat shape can be manufactured.

The flatness failure that occurs when the hot steel sheet is accelerated cooled is caused by the nonuniform temperature in the surface of the steel sheet caused by the accelerated cooling, and the heat generated in the lengthwise direction of the steel sheet after the accelerated cooling reaches the room temperature. Buckling deformation due to stress. Regarding the thermal stress in the length direction of the steel sheet, if the straightening is performed by the hot straightening machine after the accelerated cooling, the stress generated up to that point is released. Can be predicted by Therefore, (i) By measuring the in-plane temperature distribution of the steel sheet at the end of the straightening by the hot straightening machine, it is possible to calculate the thermal stress newly generated from the end of the straightening to the room temperature. (Ii) The critical buckling stress which is the criterion for buckling depends on the residual stress distribution shape. Therefore, from the distribution of residual stress obtained by the above calculation, the critical buckling stress is obtained,
By comparing with the value of the residual stress, it is possible to determine whether or not the flatness defect occurs when cooled to room temperature. (Iii) When the occurrence of buckling is predicted by the above determination, the in-plane temperature distribution of the steel sheet at the hot straightening position is calculated backward so that the residual stress distribution becomes equal to or less than the critical buckling stress distribution, and By increasing / decreasing the amount of cooling water in the width direction of the rapid cooling device according to the difference from the in-plane temperature distribution measured in i), it is possible to suppress the temperature unevenness in the width direction of the steel plate within an appropriate range, and then cool. It is possible to prevent the flatness of the steel sheet to be flattened.

In the present invention, the reason why the surface temperature distribution of the steel sheet is measured at the inlet or the outlet of the hot straightening machine following the accelerated cooling is that the residual stress generated in the steel sheet in the previous steps is released by the hot straightening. Therefore, the thermal stress that causes the flatness failure of the steel plate is determined only by the temperature unevenness at the end of hot straightening. Normally, the straightening of the steel sheet is carried out while reciprocating the steel sheet in the hot straightening machine, so the temperature of the steel sheet can be measured at a time close to the end of the straightening. It does not matter whether it is on the entry side or the exit side. The reason for calculating the residual stress is that, as will be described later, the critical buckling stress changes with the residual stress distribution, and therefore the residual stress distribution is necessary to predict the occurrence of buckling.

The reason for measuring the temperature of the entire upper surface of the steel sheet is as follows.
This is because the widthwise temperature distribution of the steel sheet is averaged in the lengthwise direction of the steel sheet to obtain the average widthwise temperature distribution. In addition, in a general accelerated cooling and hot straightening line, the accelerated cooling device and the hot straightening machine are arranged with a sufficient interval so that a steel sheet with a long length can be manufactured. By the time the steel sheet is conveyed from the cooling device to the hot straightening machine, the temperature distribution in the steel sheet in the plate thickness direction becomes substantially uniform, so it is sufficient to measure the upper surface temperature of the steel sheet. When measuring the upper surface temperature of the steel sheet, it is a good idea to use a surface thermometer such as thermography. With a surface thermometer, the temperature of the entire upper surface can be measured instantly, but with a spot thermometer or a type of thermometer that scans the spot thermometer in the plate width direction, the temperature distribution in the plate width direction of the conveyed steel plate Is difficult to measure with sufficient precision and accuracy. The accuracy of the surface thermometer depends on how many pixel points the surface thermometer has in the width direction and the length direction of the steel sheet. When putting the entire steel plate into the visual field range of the surface thermometer, width 2 to 4.5 m, length 20 to 30
It is possible to set about 50 to 130 pixels in the width direction and about 300 to 400 pixels in the length direction for a steel plate of m, which is sufficient for calculating the residual stress.

The reason why the widthwise temperature distribution averaged in the lengthwise direction of the steel sheet is obtained is that the widthwise temperature distribution of the steel sheet is not necessarily constant in the lengthwise direction of the steel sheet. Buckling of steel plate is
This does not occur due to the local distribution of the longitudinal stress in the width direction at each position in the longitudinal direction, and does not occur unless the lateral distribution of the longitudinal stress is continuous to some extent in the longitudinal direction.
Therefore, in order to consider this influence, the temperature distribution in the width direction (y direction) is averaged in the length direction, and from this average width direction temperature distribution T _AVE (y), the average width direction stress in the length direction is calculated. This is because obtaining the distribution and performing buckling evaluation is the most preferable method in order to secure the accuracy of buckling evaluation and shorten the calculation time. However, since the temperature of the front and rear regions of the steel sheet in the transport direction tends to drop extremely because cooling is also performed from the end faces, it is general to determine the average width direction temperature distribution excluding these regions. Is. The length of the front and rear edges of the steel plate excluded from the calculation of the average width direction temperature distribution is
Since the length range in which a rapid temperature decrease occurs due to the rapid cooling of a steel plate having a plate width of 1 m to 4.5 m is about 0.5 to 2 m, the length is 1/2 of the plate width.

The calculation of the width direction (y direction) distribution of the residual stress σ in the length direction (x direction) of the steel sheet is such that elastic buckling deformation occurs before composition deformation in the cooling process after the hot straightening machine. Therefore, the elastic stress calculation is sufficient, and the steel plate is divided into steel plates having a small width dy _{i in} the width direction, and the temperature difference T (y) (= T _AVE (y ) -RT)
It has been conventionally known that the residual stress distribution can be calculated by using, for example, the following calculation formula. σ (y _i ) = α Ε Τ (y _i ) −1 / W · Σα Ε Τ (y _i ) dy _i −12 y _i / W ³ Σα Ε Τ (y _i ) y _i dy _i (1) where E is Young's modulus, α is a coefficient of linear thermal expansion, σY is a yield stress of a steel plate, W is a plate width, and Σ is a sum of steel plates having all minute plate widths. The occurrence of buckling is predicted from the obtained residual stress distribution as follows. In the residual stress distribution generated in the steel sheet, when the number of compressive stress regions is one, the width B of the compressive stress region is calculated as B = Ba, as shown in FIG. When a plurality of compressive stress areas are mixed, the tensile stress area sandwiched between the compressive stress areas is considered to be an equivalent compressive stress area, and the width B is B as shown in FIG. 8B.
= Bb. If the compressive stress region includes the width end,
(C) As shown in the figure, the width twice the actual compressive stress area Bc is defined as the compressive stress area width B = 2Bc. Using the average compressive stress σc σc = 1 / BΣcσ (y _i ) dy _i (2) in the above equivalent compressive stress region, it is defined that buckling occurs if | σc | ≧ σ _CR (3). Where Σ
c is the sum of only compressive stress (σ (y _i ) is negative). When the buckling occurrence is defined by the equations (2) and (3), the inventors can approximate the value of the critical buckling stress σ _CR by the following equation based on the result of the buckling analysis using the finite element method. I found it. _{σ CR = (K 1 + KΔ} ) Eπ 2/12 (1-ν 2) · (t / B) 2 (4) equation, where, K ₁ is an buckling stress coefficient when compressive stress zone is one , KΔ is a buckling stress coefficient when there are a plurality of compressive stress regions (including a tensile stress region in the equivalent compressive stress region), t is a plate thickness, π is a circular constant, and ν is a Poisson's ratio. The buckling stress coefficient K _{1 in} the case of one compressive stress region is expressed by the following equation. K ₁ = 0.085 (B / W) ^0.9 · exp (5.8d _G / W) (5) where d _G is the smaller distance from the edge of the plate width to the center of gravity of the equivalent compressive stress region. Represent The buckling stress coefficient KΔ in the case where there are a plurality of compressive stress regions is KΔ = 12 (1-ν ² ) B ² / {Eπ ² 24 ² } · (0.71β-1.6) (6) Equation β = 1 / BW · Σ {(d _G −d _i ) · exp (5.8 d _G / W) · ∫ _bi σdy} (7) Expression Here, d _i represents the center of gravity of the i-th compressive stress region or the tensile stress region, and ∫ _bi dy represents the integral in the i-th compressive or tensile stress region. Above (4)
The value of the critical buckling stress σ _CR calculated by the equations (7) shows a good agreement with the result obtained by the finite element method as shown in FIG. 9. When actually applying the buckling criterion of the formula (3), considering the safety factor, the critical buckling stress defined by the formulas (4) to (7) is multiplied by 0.9. Is used as the critical buckling stress.

Since the buckling of steel sheets often occurs in batches for each lot, when the occurrence of buckling is predicted for one steel sheet, the residual stress of the next steel sheet should be below the critical buckling stress. It is necessary to control the amount of rapid cooling water, control the temperature distribution in the width direction at the end of hot straightening, and prevent buckling. As a result of investigation by the present inventors, the temperature distribution in the width direction of the steel sheet is about 250 mm when the distance between the high temperature portion and the high temperature portion or the distance between the low temperature portion and the low temperature portion is the smallest.
Met. If there are three nozzles in the meantime, by adjusting the amount of water, it becomes possible to perform cooling while suppressing the occurrence of temperature unevenness. Therefore, the required nozzle pitch is about 125 mm in the width direction of the steel sheet. The width of the rapidly cooled steel plate is
It is usually 2 to 5 m, and the lower limit of the number of nozzles in the width direction is set to 1/15 of the plate width and the upper limit thereof is set to 1/40.

The first method for modifying the temperature distribution so as to prevent buckling (hereinafter referred to as temperature modifying method 1) is the predicted ratio r of critical buckling stress to residual stress r, r = σ _CR / | Σc | (≦ 1) Equation (8) is calculated, and as shown in FIG. 10, the average temperature of the steel sheet (−T
_AVE ) and the temperature at each position y in the width direction in the average width direction temperature distribution T _AVE (y), ΔT (y) (= T _AVE (y) −
(T _AVE ) is to reduce the average temperature distribution in the width direction as a whole so that the temperature becomes less than r times that when buckling occurs. In order to reduce the average temperature difference in the width direction, in the cooling device in which the cooling water amount is variable in the width direction, the water amount from the nozzle in the relatively high temperature part is increased and the water amount in the relatively low temperature part is decreased. Just go. Increase / decrease in water quantity of each nozzle ΔQ
_i is determined as follows from the average value Q _AVE of the water amount of each nozzle in the width direction when the buckling is determined and each spray water amount Q _i . Generally, in rapid cooling of a steel sheet, from the viewpoint of metallographic structure, in order to control the temperature of the steel sheet at the end of cooling to a predetermined temperature, the cooling water flow rate is set for each steel sheet thickness or the temperature of the steel sheet at the start of cooling. The relationship between the temperature and the steel plate temperature at the end of cooling has been investigated. Similarly, the relationship between the average amount of cooling water per nozzle and the average temperature of the steel plate in the hot straightening part is shown in FIG.
As shown in (a) and (b), the thickness of the steel sheet, the average temperature of the steel sheet at the start of cooling, and the transport speed are obtained in advance. Than this,
Rate of change of average temperature of steel plate according to change in water amount Δ ￣ T _AVE / Δ
With Q _AVE, from _{ΔQ i = (1-r)} (T AVE (y) -¯T AVE) ÷ (Δ¯T AVE / ΔQ AVE) 9 formula, it is possible to obtain the flow rate change amount of each spray .

The second method for correcting the temperature distribution so as to prevent buckling (hereinafter referred to as "temperature correction method 2") is a residual stress in which the buckling is less likely to occur even if the same temperature difference in the width direction occurs in the steel sheet. This is a method of correcting the shape of the temperature distribution in the width direction, that is, the position and width of the relative low temperature portion so that the distribution becomes a distribution. FIG. 12 is a diagram showing the relationship between the critical buckling stress and the position of the compressive stress when two compressive stresses are symmetrical in the width direction of the steel sheet. It can be seen that the critical buckling stress becomes maximum when the compressive stress regions are at positions of 1/4 of the plate width, and buckling is most difficult. Therefore, locating the relatively low temperature portion at a position of 1/4 of the plate width is a method of making buckling less likely to occur when the same temperature difference in the width direction occurs in the steel plate. Usually, in the rapid cooling of steel sheets, the lower limit of the temperature deviation of the steel sheet is often specified not only by buckling but also by metallographic requirements. Therefore, for example, when the lower limit of the temperature deviation due to the metallographic requirement is set to 40 ° C., the temperature deviation is 4
Conditions for temperature distribution that does not cause buckling even at 0 ° C. are obtained as illustrated in FIGS. 13 (a) and 13 (b). FIG.
(A) and (b) show the width of the relative low temperature region for preventing buckling as a function of the position of the center of gravity of the relative low temperature part when the temperature difference is 40 ° C. It is shown that when the positions of the relative low temperature parts are located at ¼ of the plate width, the buckling is less likely to occur even if the width of the low temperature region is wider than in the case of being located at other places. The temperature distribution satisfying the buckling prevention temperature condition shown in FIGS. 13 (a) and 13 (b) is set to a target value T _OBJ (y).
Then, the increase / decrease amount ΔQ _i of the water amount of each nozzle for setting the average width direction temperature distribution T _AVE (y) to T _OBJ (y) is ΔQ _i = (T _OBJ (y) −T _AVE (y)) ÷ (ΔT _AVE / ΔQ _AVE ) It is given by the equation (10).

Based on the equation (9) of the temperature correction method 1 or the equation (10) of the temperature correction method 2, the flow rate of the nozzle connected to each nozzle of the cooling device and the flow rate adjusting valve allow the nozzle flow rate to be in an appropriate range. By controlling inside, the buckling prevention average width direction temperature distribution can be achieved and the shape of the steel sheet can be kept flat. Note that the buckling prevention by the temperature correction method 1 and the temperature correction method 2 is effective from the next steel plate determined to cause buckling. Therefore, for a steel plate that is determined to be buckled, if it is cooled to room temperature as it is, buckling will eventually occur, so before the buckling occurs, warm correction is performed again to release the residual stress. It is preferable to prevent buckling. At the time of cooling, the widthwise temperature difference generated by the rapid cooling also decreases as the average temperature of the steel sheet decreases, but the average temperature of the steel sheet and the widthwise temperature difference at this time are in a substantially proportional relationship. Since the compressive residual stress increases linearly with the decrease of the temperature difference in the width direction, the steel plate average temperature ￣ T _AVE 'when the residual stress reaches the critical buckling stress is given as follows (Fig. 14 ( a) (b)). ￣ T _AVE '= R. T. _{+ R (¯T AVE -R.T.) (} 11) formula, where, r is the ratio between the residual stress and the critical buckling stress predicted given by equation (8). It is possible to prevent buckling by correcting the average temperature of the steel sheet −T _AVE before it becomes less than −T _AVE 'given by the equation (11) by the leveler again.

[0027]

【Example】

Example 1 Details of the method of the present invention will be described below with reference to FIGS. 1 and 2 showing an example of an embodiment. FIG. 1 is a schematic configuration diagram of a steel plate rapid cooling line for carrying out the method of the present invention, and FIG. 2 is an explanatory diagram of a nozzle header row arrangement for controlling the amount of cooling water at a predetermined pitch in the steel plate width direction. In FIG. 1, 1 is a steel plate, 2 is a hot rolling mill, 3 is a rapid cooling device capable of controlling the temperature in the strip width direction, 4 is a hot straightening machine, and 5 is a thermography provided at the upper inlet of the hot straightening machine 4. Is a surface thermometer, and 6 is a thermometer provided on the upper and lower surfaces of the hot rolling mill 2 on the delivery side. 7 is a cooling control unit, 8 is a cooling water amount adjusting unit, and the cooling control unit 7 is
The upper and lower surface temperatures of the steel plate 1 input from the thermometers 6 and 6 are compared with a predetermined cooling start set temperature, and the hot rolling mill 2 to the forced cooling device 3 are controlled so that the cooling start temperature becomes the cooling start set temperature. The transport time is controlled, and the cooling start temperature is controlled to the cooling start set temperature. In addition, the cooling control unit 7 uses the surface thermometer 5
The temperature distribution in the width direction is obtained based on the surface temperature of the steel sheet 1 input from the above, the temperature distribution in the width direction (y direction) is averaged in the length direction, and this average width direction temperature distribution T _AVE (y) is predetermined. Compared with the average set temperature in the width direction, the correction amount of the cooling water amount in the width direction is calculated, and the cooling water amount adjusting unit 8 is instructed to control the cooling water amount injected from the rapid cooling device 3 to the upper and lower surfaces in the plate width direction. The temperature distribution in the width direction of the steel plate 1 is controlled to be a predetermined temperature. The steel sheet 1 cooled to a predetermined temperature by the rapid cooling device 3 is hot-corrected by the hot-correcting machine 4 and then conveyed to the next step.

As shown in FIG. 2, the rapid cooling device 3 has four pipe laminar headers 22 each having a nozzle 21 arranged so that the distance Wa is 125 mm, although the pipe laminar header 22 is not shown along the conveying direction of the steel sheet 1. In the plate width direction, the distance W between the nozzles 21 of the adjacent pipe lamina headers 22 is arranged.
36 rows in the width direction so that b is 125 mm, a total of 144
The pipe laminar header 22 of the book was arranged. Although not shown in the drawings, a flow control valve and a flow meter are connected to each pipe lamina header 22, and the cooling water amount adjusting unit 8 adjusts the amount of cooling water to the 36 pipe laminar headers 22 in the width direction to 125 mm in the plate width direction. It is configured so that the amount of cooling water can be controlled by the pitch. The total cooling water supply capacity is 200 Ton / min, and the total number of nozzles 21 is 5760, so the maximum amount of water per nozzle 21 is about 34.7 liters / min.
Minutes.

Further, the cooling control section 7 determines, based on the surface temperature of the steel plate 1 inputted from the surface thermometer 5, 50 m in the plate width direction.
Interpolation calculation is performed again in the form of a grid with m pitches and 500 mm pitches in the plate length direction, and then the average temperature distribution in the width direction, residual stress, critical buckling stress are calculated, and the presence or absence of buckling is determined. The time required for the transfer processing of these temperature data and the buckling evaluation from the temperature data is approximately 10 to 10 per steel plate.
It was 15 seconds. Further, when the cooling control unit 7 determines that buckling has occurred, the cooling control unit 7 calculates a correction amount of the cooling water amount in the width direction of the steel plate 1 and instructs the cooling water amount adjusting unit 8 to instruct the nozzles 21 of the predetermined row of the pipe laminar header 22. It is configured to control the amount of cooling water from the indicated value.

With the above configuration, the cooling control unit 7 compares the upper and lower surface temperatures of the steel sheet 1 input from the thermometers 6 and the predetermined cooling start set temperature, and the cooling start temperature starts cooling. The conveyance time from the hot rolling mill 2 to the forced cooling device 3 is controlled so as to reach the set temperature, and the cooling start temperature is controlled to the cooling start set temperature. In addition, the cooling control unit 7
Is based on the surface temperature of the steel plate 1 input from the surface thermometer 5 and has a pitch of 50 mm in the plate width direction and 500 m in the plate length direction.
After the interpolating calculation is again performed for the m-pitch grid pattern, the average width direction temperature distribution, residual stress, critical buckling stress are calculated, and the presence or absence of buckling is determined. When it is determined that buckling occurs,
The correction amount of the cooling water amount in the width direction of the steel plate 1 is calculated, and the cooling water amount adjusting unit 8 is instructed to control the cooling water amount from the nozzles 21 of the predetermined pipe laminar header 22 row to the instructed value. Therefore, the temperature of the steel sheet 1 in the width direction and the longitudinal direction can be suppressed within a range of a normal error (± 5 ° C.), and the temperature unevenness that occurs during rapid cooling of the hot steel sheet causes the temperature of the steel sheet to drop to room temperature. The presence or absence of buckling deformation occurring in the process can be immediately predicted, and the steel plate 1 determined to have no buckling deformation can be immediately transferred to the refining process,
The steel plate 1 determined to have buckling deformation can be immediately transferred to the re-correction step. Further, regarding the steel plate 1 that undergoes rapid cooling after the steel plate 1 determined to undergo buckling deformation, the cooling water amount in the steel plate width direction in the rapid cooling is set so that the temperature distribution of the steel plate 1 is within the buckling occurrence limit. By controlling, it is possible to reliably prevent the occurrence of buckling deformation. In this embodiment, the surface thermometer 5
Although the above is provided above the inlet of the hot straightening machine 4, providing the above above the outlet of the hot straightening machine 4 does not affect the effect of the present invention. Moreover, the upper surface temperature of the steel plate 1 was measured by the surface thermometer 5,
A thermometer is also installed on the lower surface of the steel plate 1 to confirm that there is no difference in the upper and lower surface temperatures of the steel plate 1.

Example 2 Using the rapid cooling device of Example 1 above, the method of the present invention was applied to the steel plate of Table 1 and No. 1 of Table 1. The measurement was carried out under the conditions of 1 to 5, and the average deviation ΔQi of the cooling water amount of one nozzle in the steel plate width direction, the actual cooling end temperature, the average temperature deviation ΔT _AVE , and the flatness failure rate when cooled to room temperature were measured. The results are shown in Table 2 together with the comparative example. In addition, No. For each condition 1 to 5,
The method of the present invention and the comparative method were carried out on 100 to 200 steel plates. In the method of the present invention, the flatness defect occurrence rate is the ratio of the number of plates determined to be flatness to the total number of plates under each condition, and in the comparative example, the flatness is actually flat against the total number of plates under each condition. It was calculated by the ratio of the number of plates with defects.

[0032]

[Table 1]

[0033]

[Table 2]

As shown in Tables 1 and 2, by applying the method of the present invention, in the comparative example in which the temperature control in the width direction is not performed, the flatness defect is as high as 41% even if the flatness defect occurs. The occurrence rate could be suppressed to 3% or less. Further, the cooling end temperature is usually within the error range (± 5) even when the temperature control by the width direction cooling water amount control is performed.
The temperature could be suppressed to the range of (° C). In addition, regarding the steel sheet determined to have a flatness failure by the method of the present invention, before being cooled to room temperature, the steel sheet is actually subjected to warm straightening again under the condition shown in Formula 11 It was possible to prevent flatness from occurring.

[0035]

As described above, according to the method of the present invention, due to the temperature unevenness generated during the rapid cooling of the hot steel sheet,
The presence or absence of buckling deformation that may occur in the process of cooling the steel plate down to room temperature can be predicted immediately after rapid cooling.Therefore, if the steel plate is determined to have no buckling deformation, transfer it immediately to the refining process. In addition to the above, the steel sheet determined to have buckling deformation can be immediately transferred to the re-correction step. In addition, for the steel plate that is subjected to rapid cooling after the steel plate determined to cause buckling deformation,
By controlling the amount of cooling water in the width direction of the steel plate in the rapid cooling so that the temperature distribution of the steel plate is within the buckling occurrence limit, it is possible to reliably prevent the buckling deformation.

[Brief description of drawings]

FIG. 1 is a configuration diagram when a method of the present invention is applied to a hot steel sheet cooling line.

FIG. 2 is an explanatory perspective view of an arrangement of nozzle header rows for controlling the amount of cooling water at a predetermined pitch in the width direction of the steel plate.

FIG. 3 shows the temperature distribution in the width direction of the steel sheet measured in the hot straightening part after rapid cooling and the distribution of the residual stress in the width direction of the steel sheet after the hot straightening steel sheet was naturally cooled to room temperature. An example is shown, (a) is a graph showing the relationship between the sheet width direction distance from the machine side (MS) and the steel sheet temperature before hot straightening, and (b) figure is the sheet width direction distance and internal stress It is a graph which shows a relationship.

FIG. 4 shows an example of temperature distribution in the width direction of a steel plate,
Figure (a) shows the temperature distribution and residual stress distribution when the relative low temperature region exists in only one location in the width direction, and (b) shows the critical buckling when there is only one compressive stress distribution in the width direction. It is a figure which shows the relationship between a stress and a compression stress position.

FIG. 5 is a graph showing the relationship between the critical buckling stress and the compressive stress position when the compressive stress distribution exists only at one location in the width direction.

FIG. 6 shows that the critical buckling temperature difference (ΔTmax) _CR in a temperature distribution that causes a plurality of compressive stress regions varies depending on the temperature distribution. FIG. ) The figure shows the increase in the critical buckling temperature difference (ΔTmax) _CR due to the temperature distribution in the width direction.

FIG. 7 shows the difference in critical buckling temperature difference (ΔTmax) _CR due to the difference in position in the width direction in the low temperature region (compressive stress region),
FIG. 7A is a diagram showing a case where the low temperature region is near both ends, and FIG. 9B is a diagram showing a case where the low temperature region exists in the width direction ¼.

FIG. 8 is a diagram for explaining the definition of how to obtain the width B of the compressive stress region. FIG. 8 (a) shows that when the compressive stress region is one in the width direction,
(B) The figure shows (c) when there are multiple compressive stress regions in the width direction.
The figure shows a case where the compressive stress region includes the width end portion.

FIG. 9 is a graph showing the relationship between the critical buckling stress expressed by equations (4) to (7) and the critical buckling stress obtained by buckling analysis by the finite element method.

FIG. 10 is a diagram for explaining temperature distribution correction by temperature correction method 1.

FIG. 11 is a graph showing the relationship between the transport speed, the steel plate average temperature, and the average cooling water amount, and FIG. 11A is a graph showing the relationship between the transport speed, the steel plate average temperature, and the average cooling water amount per nozzle; b) The figure is a graph showing the relationship between the average amount of cooling water per nozzle and the average steel plate temperature.

FIG. 12 shows the relationship between the critical buckling stress and the position of compressive stress when two compressive stresses are symmetrical in the width direction,
FIG. 5A is a graph showing the relationship between the critical buckling stress and the compressive stress with the ratio of the low temperature region width to the plate width, and FIG. 8B is a diagram showing the position of the low temperature region width.

FIG. 13 shows conditions of temperature distribution that does not cause buckling of a steel plate having a thickness of 10 mm at a temperature deviation of 40 ° C.
(A) is a diagram showing the widthwise distance and temperature distribution of the steel plate,
FIG. 6B is a graph showing the ratio of the low temperature region width to the plate width and the distance from the steel plate end.

FIG. 14 is an explanatory diagram of how to determine a steel plate average temperature −T _AVE 'when the residual stress of the steel plate reaches the critical buckling stress in the cooling process, and FIG. 14A shows the plate material average temperature and the plate width direction temperature. A graph showing the relationship with the deviation, and FIG. 6B is a graph showing the relationship between the plate width average temperature and the compressive residual stress.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel plate 2 Hot rolling mill 3 Rapid cooling device 4 Hot straightening machine 5 Surface thermometer 6 Thermometer 7 Cooling control unit 8 Cooling water amount adjusting unit 21 Nozzle 22 Pipe lamina header

Claims (2)

[Claims] 1. A method for cooling a hot steel sheet in a step of accelerating cooling while transporting the hot steel sheet in a longitudinal direction and then performing hot straightening, wherein a nozzle header row in which nozzle headers are arranged at predetermined intervals in a longitudinal direction of the steel sheet is provided. A cooling device having a flow rate adjusting mechanism capable of controlling the amount of cooling water from the injection nozzles of each nozzle header row is provided at a predetermined interval in the width direction, and a steel plate is introduced at the inlet or the outlet of the hot straightener following the cooling device. The temperature distribution of the entire surface is measured, the average width direction temperature distribution of the entire upper surface of the steel sheet is compared with a predetermined set value, and the amount of cooling water to each nozzle header row of the cooling device is controlled. How to cool hot steel plates. 2. A method of cooling a hot steel sheet in a step of accelerating cooling while transporting the hot steel sheet in a longitudinal direction and then hot-correcting the hot steel sheet, wherein a nozzle header row in which nozzle headers are arranged at predetermined intervals in the longitudinal direction of the steel sheet is provided. A cooling device having a flow rate adjusting mechanism capable of controlling the amount of cooling water from the injection nozzles of each nozzle header row is provided at a predetermined interval in the width direction, and a steel plate is introduced at the inlet or the outlet of the hot straightener following the cooling device. The temperature distribution of the entire surface is measured, and the average width direction temperature distribution is obtained by averaging the width direction temperature distribution at each position in the length direction of the steel sheet based on the temperature distribution of the entire steel plate upper surface. Prediction of residual stress distribution and critical buckling stress after virtual steel sheet having continuous directional temperature distribution in the longitudinal direction after being left to cool to room temperature is calculated and based on the predicted residual stress and critical buckling stress. The presence or absence of buckling deformation at room temperature is determined, and if buckling deformation occurs, the in-plane temperature distribution of the steel plate at the inlet or outlet of the hot straightening machine should be adjusted so that the residual stress is less than the critical buckling stress. Is calculated back, in accordance with the difference between the measured temperature distribution in the steel plate surface of the inlet or outlet of the hot straightener, to calculate the correction instruction value of the cooling water amount of each nozzle header row in the width direction of the cooling device, A method for cooling a hot steel sheet, comprising controlling the amount of cooling water to each nozzle header row of the cooling device.