KR100353283B1 - Method and apparatus for manufacturing hot-rolled steel sheet - Google Patents

Abstract

A method of manufacturing a hot-rolled steel sheet which can precisely predict a shape change of a hot-rolled steel sheet in accordance with a difference in shape during air-cooling and a difference in a stacking method, or a shape change after cutting, in advance, And a manufacturing apparatus.

In the manufacturing method of the present invention, the steel plate subjected to the accelerated cooling process or the hot calibration process is air-cooled. The temperature distribution of the plate surface of the steel plate is measured before air cooling, the residual stress of the steel plate produced after air- And a method of manufacturing a hot-rolled steel sheet in which a shape change of a steel sheet is predicted from a process condition of the steel sheet, and a cold-proofing process and / or a heat treatment is performed on the steel sheet determined to be defective, By measuring the flatness of the surface of the steel sheet before or after the measurement of the plate surface temperature distribution of the steel sheet and correcting the amount of thermal strain calculated from the distribution of the residual stress to the amount of strain after the air- Predict shape changes.

Description

Method and apparatus for manufacturing hot-rolled steel sheet

The present invention relates to a method for manufacturing a hot-rolled steel sheet and an apparatus for manufacturing the same. More specifically, by precisely predicting the shape change of the hot-rolled steel sheet during air cooling and the shape change due to the difference in the place of cutting, or the shape change after cutting, and appropriately correcting processing conditions accordingly, And to a manufacturing method and a manufacturing apparatus which can prevent the above-described problems.

The hot-rolled steel sheet is generally produced by hot rolling to a predetermined steel sheet size, followed by accelerated cooling and quenching, and then calibrated by a hot leveler. The steel sheet thus manufactured is shipped as it is when it is judged to be flat in on-line shipment inspection, and shipped after it is calibrated and flattened by a cold leveler or the like when the shape defect remains.

However, there is a possibility that defects such as warpage and wave shape (or waveform) may occur during the process of distribution or in the process of changing the stacking method of the steel sheet after shipment at the time of shipment. It has also been pointed out that, when a steel plate is subjected to heat treatment such as gas cutting, defects such as warping and a waveform may occur.

In addition, when such a defective shape occurs when the steel sheet is to be placed in place or after the cutting process is performed, it is necessary to perform an operation in an unlearned state called straightening of the strain, resulting in a decrease in productivity, Depending on the strength and dimension of the steel plate, it may not be corrected due to the limitation of the calibration ability.

Such a defective shape is known to be caused by residual stress in the steel sheet. The shape defects when the method of changing the steel sheet is changed is a steel sheet in which the residual stress level is in the vicinity of the buckling critical stress, and on the table roll, the self weight of the steel sheet and the restraint by the table roll are combined, . However, if the post-tensioning method is changed, the restraint state and the residual stress state change, buckling deformation occurs, and a defective shape occurs. In addition, the shape defect occurring after cutting may be caused by the gas cutting condition, but the residual stress unevenly distributed inside the steel sheet at the time of cutting may be released, resulting in warping and transverse bending.

Particularly, in the accelerated cooling type steel plate, uneven residual stress is easily generated in the steel plate due to a temperature deviation due to uneven cooling in the plate surface during acceleration cooling, and the residual stress state is close to the buckling critical stress This tendency is remarkable because there are relatively many cases.

Therefore, in order to select the steel plate which buckles due to the difference in the method of putting it in the past, the flatness is checked with the eye in a state of being placed on a square bar or in a state of hanging down by a crane. However, it is difficult to completely identify a shape-unstable steel sheet because the shape of the steel sheet varies depending on the method of putting it on a square bar, for example, in such a snow check. In addition, since the shape of the crane hangs down due to its own weight, it is difficult to accurately identify the waveform due to buckling. Therefore, it has been pointed out that such a method is difficult to perform reliable identification. Further, in these methods, it is necessary to place the steel plate in the form of each bar on the basis of the shipment determination and to lift the steel plate by a crane, thereby deteriorating the productivity. However, these methods can not predict the shape defects that occur after cutting.

Japanese Patent Publication Nos. 4-8128 and 4-8129 disclose a method of estimating residual stress by measuring the surface temperature distribution of the plate material and estimating and calculating the amount of deformation after adjustment processing have. The inventors of the present invention have also found that the buckling in the flatness of a steel sheet can be easily and accurately predicted by a FEM analysis by a large computer, that is, a method that can be predicted simply without performing a hot-point carbon-carbon segregation analysis by the finite element method Have been proposed previously. (Japanese Patent Laid-Open No. 8-187505)

Based on such a prediction technique, it is possible to provide a steel sheet which does not cause a defective shape even when a large-sheet steel sheet is shipped at the time of shipment and cut by the user at the time of use. However, even with these prediction techniques, it is not possible to completely prevent the defective shape, and there is room for improvement, and it is desirable to develop a prediction technology with a higher precision, and even if the shipment inspection is failed, Techniques that can be accepted products have been desired.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a hot-rolled steel sheet which can precisely predict a shape change due to a difference between a shape change and an air- And a method for manufacturing the hot-rolled steel sheet.

1 is a schematic view showing a corrugated shape of a steel sheet, (a) is a sectional view, and (b) is a perspective view.

Fig. 2 is a block diagram showing a preferred apparatus configuration for determining a defective shape.

3 is a schematic explanatory diagram as an example of a preferred manufacturing line of the present invention.

FIG. 4 is an explanatory diagram of a coordinate system in the derivation process of the buckling simple predictive equation (buckling simple predictive equation) according to the present invention.

5 is a graph for explaining the evaluation result of the precision of the above-mentioned buckling prediction equation.

Fig. 6 is an explanatory view of the evaluation result on the accuracy of the conventional buckling-only prediction equation.

Fig. 7 is a graph showing the residual stress distribution as the panel width direction.

8 is a graph showing the relationship of residual stress before and after calibration.

9 is a graph showing the relationship between the residual stress before calibration and the reduction coefficient of residual stress due to calibration.

10 is a graph showing the residual stress distribution in the direction of the width of the accelerated cooling steel sheet.

11 is a graph showing the residual stress distribution in the plate width direction after quenching.

12 is a graph showing the relationship between the press-in amount (push-in amount) and the residual stress of the press calibration.

13 is a graph showing stress distribution in the plate thickness direction at the time of accelerated cooling stop.

FIG. 14 is a graph showing a temperature rise characteristic of a treated steel sheet.

15 is a graph of the residual stress after heat treatment.

16 is a temperature distribution diagram in the direction of the width in the case of measuring the strain amount of two kinds of the rolled steel sheets.

According to the manufacturing method of the present invention which solves the above problems, in the air-cooling of a steel sheet subjected to an accelerated cooling step or a hot calibration step, the temperature distribution of the surface of the steel sheet is measured prior to the air cooling, the residual stress of the steel sheet, As a result of predicting the shape change of the steel sheet from the distribution condition of the residual stress and the processing condition of the steel product sheet, the hot rolled steel sheet determined to cause the shape defect is subjected to cold correction treatment and / or heat treatment, A method for producing a hot-rolled steel sheet. Further, the present invention is a method for measuring the flatness of the surface of a steel sheet before or after measurement of the surface temperature distribution of the steel sheet, measuring the amount of heat strain (mature strain amount) calculated from the calculated distribution of the residual stress, And the shape defect is removed in advance from the prediction of the shape change by correcting the amount of strain after air cooling calculated from the result.

Examples of the defective shape include defects such as buckling, defects such as poor flatness or transverse bending after cutting, and when performing a cold-calibrating process or a heat treatment on a steel plate which is judged to have any of the shape defects, And the shape change after the treatment is predicted from the residual stress at the time of performing the cold calibrating treatment and / or the heat treatment according to each treatment condition, and as a result, the treatment condition in which any shape defect does not occur is selected. Also, when the flatness is measured, when the shape of the steel sheet has the height of the pitch p and the elongation percentage difference?? _WAVE at the center of the width of the plate and the end of the plate and DELTA? _TH of the thermal strain in the width direction of the plate, The amount of strain: δ 'can be calculated by the following equation.

The hot-rolled steel sheet manufacturing apparatus for performing the cold-calibrating process and / or the heat-treating process on the steel sheet determined to have the defective shape includes plate surface temperature measuring means for measuring the plate surface temperature distribution of the steel sheet, Calculating means for predicting a shape change amount of the steel sheet after air cooling based on the flat surface temperature distribution data obtained by the flat surface temperature measuring means and the flatness data obtained by the flatness measuring means, (Determination means) for comparing the shape change amount with a predetermined determination criterion to determine whether or not the steel sheet has passed the shape change amount; and a control means for controlling the shape of the steel sheet from a plurality of processing conditions previously set in relation to the cold- And a process condition selecting means for selecting a process condition for suppressing the change amount within the acceptance criteria It is recommended that you employ the device.

The following determination method [A] [B] may be employed for the buckling deformation caused by the change in the method of placing the steel sheet during the defective shape.

(i) Judgment method [A]

The cause of the defective shape according to the change of the method of placing the steel sheet depends on whether or not the deformation of the steel sheet is constrained by the influence of its own weight irrespective of whether the residual stress exceeds the buckling critical stress. Therefore, it is possible to predict the buckling critical stress, and to identify the shape-unstable steel plate even if the residual stress exceeds the buckling critical stress. Therefore, it is recommended to adopt the following judgment method.

That is, when assuming that the residual stress distribution calculated from the measurement result of the plate surface temperature of the steel sheet is? _Act (x, y), in predicting the shape change from the residual stress distribution, region Ω ₁ and the other region Ω ₂ waro split and buckling threshold in the region Ω ₁ in the state average σ _cr of the stress ^(-), and calculates the following expression (1), (2) and buckling when expression is satisfied And if it is not established, it is determined that buckling does not occur.

(Equation 1)

Σ ₀ : Constant determined according to the steel plate size, buckling mode, area division,

F (xi, yj): the number of irregularities determined by the steel plate size, buckling mode,

σ act ^(-) : average of σ _act in the region Ω ₁

σ act ⁽⁺⁾ : average of σ _act in the region Ω ₂

N ₁ , N ₂ : a set of all points (xi, yj) in each region Ω ₁ , Ω ₂

The average of the residual stress on the plate is divided into a compression zone Ω ₁ and another zone Ω _2, and the average σ _cr ^(-) of the stress in the zone Ω ₁ in the buckling critical state is defined as the buckling critical stress. (1), considering the residual stress distribution in consideration of the buckling critical stress? ₀ corresponding to the residual stress distribution close to the rectangular shape as shown in the formula (1) 2, 3], it is possible to calculate σ _cr ^(-) corresponding exactly to the actual residual stress distribution. Thus, buckling determination with high accuracy can be easily performed.

(ii) Judgment method [B]

Further, from the result of the prediction of the shape change of the steel sheet, when the shape defect occurs, the residual steel sheet after the cold-calibrating process and / or the heat treatment is calculated for the determined steel sheet, I as judging whether or not, the cold calibration process and / or also calculates a residual stress distribution after performing a heat treatment and, at the same time, the average of the residual stress in the region Ω ₁ 'is the residual stress after the correction is compressed in the plate surface σ _act ^{af (-) -} if wareul calculated and, and then determined that the buckling occurs when the 3 expression is satisfied, may not be satisfied and, buckling threshold state average σ _cr ^af region Ω ₁ stress in 'of from ⁽⁾ It is preferable to adopt a method of determining that buckling does not occur.

In this method, on the basis of the residual stress distribution after the correction, the left limb critical stress? _Cr ^{af (-)} is calculated as described above, and the comparison is made to determine whether or not buckling occurs after the calibration . According to the determination result, the cold calibrating process and / or the heat treatment is performed under the condition that the steel plate does not buckle. Therefore, in this method, for example, a correction condition that does not cause buckling can be set merely by determining the degree of reduction of the residual stress for each levellers processing condition.

In addition, when performing the cold calibrating process on the steel plate with the cold leveler, it is preferable to adopt the following calibrating method [C].

(iii) Calibration method [C]

That is, the residual stress distribution before calibration is obtained, and the residual stress distribution after calibration is calculated when the obtained steel sheet with the residual stress distribution state is corrected for a plurality of predetermined downward pressing patterns. Then, according to the calculation result, A buckling judgment as to whether or not the steel plate after the calibration has buckled is performed. From the result of this determination, a pressing down pattern not to buckle is selected, and the steel plate is calibrated by the roller leveler with the selected pressing down pattern.

Specifically, after obtaining the residual stress distribution before the calibration, it is preferable to calibrate the steel plate to the cold leveler in accordance with the following procedures (1) to (4).

( ₁₎ The reduction factor? _I of the residual stress due to the calibration from the residual stress distribution before calibration is calculated for a plurality of predetermined reduction patterns? ₁ .

and? _i = a pressing-down pattern formed by the amount of bending in the inward and outward directions,

Subscript _i : Number of patterns to reduce

F _j : the number of irrigation water determined by plate thickness, plate width,

σ _b : Residual stress distribution in the plate before calibration

② Calculate the residual stress distribution σ _b after calibration from the reduction factor η _i of this residual stress to determine buckling.

(3) From this result, we select the pressing pattern γ _i that does not buckle after calibration and find the pressing force pattern γ _min and the maximum pressing force pattern γ _max that minimize the pressing force.

④ The pressing force

And at least one pass is corrected as a reduction pattern? _OPT that satisfies the relationship of?

Further, in controlling the shape change occurring when cutting the steel plate subjected to the accelerated cooling after the quenching or hot rolling in the allowable amount, the residual stress of the steel sheet is estimated, and the shape change amount at the time of cutting is calculated based on the residual stress It is preferable to determine whether or not the shape change amount exceeds the permissible amount by predicting and if the allowable amount is exceeded, it is preferable to perform the heat treatment under the conditions determined based on the residual stress and allowable amount of the steel sheet.

Further, when the steel sheet is subjected to the heat treatment and control so that the deformation amount? Of the cutting member obtained by the cutting operation satisfies the predetermined allowable amount? ₀ when cutting the steel sheet into a predetermined shape, the following heat treatment method [D] Is recommended.

(Iv) Heat treatment method [D]

That is, an estimated residual stress? ₀ before the heat treatment of the steel sheet is estimated and calculated in accordance with the manufacturing conditions of the steel sheet,

The estimated deformation amount? ₁ of the cutting member in the case where the steel sheet is not heat-treated based on the estimated residual stress? ₀ ,

It is determined whether or not the estimated deformation amount? ₁ satisfies the allowable amount? ₀ ,

When the estimated deformation? ₁ does not satisfy the allowable amount? ₀ , the estimated residual stress? ₁ when the steel sheet is heat-treated is estimated and calculated,

A heat treatment condition is determined based on the estimated residual stress? ₀ ,? ₁ and the allowable amount? ₀ of the steel sheet,

The steel sheet is subjected to heat treatment under the heat treatment conditions so as to control the deformation amount of the cutting member to satisfy a predetermined allowable amount? ₀ .

When the steel sheet is subjected to the heat treatment, the heat treatment may be performed on the conditions obtained by the following procedures (1) to

① The residual stress of the steel sheet after the nicking or accelerated cooling is calculated or set.

② Calculate the residual stress change of the steel plate by the calibration before heat treatment according to the calibration conditions.

(3) Residual stress after heat treatment is calculated according to the residual stress before heat treatment of this steel plate, the creep characteristics during heat treatment, and the condition of the surface before heat treatment.

(4) Calculate the residual stress after heat treatment and the amount of deformation by cutting from the steel sheet size after cutting.

(5) Using the above conditions (3) to (4), calculate the heat treatment conditions consisting of the heat treatment temperature and the heat treatment time at which the deformation by cutting is not more than the allowable amount.

It is premised on the assumption that the residual stress generated after air cooling substantially coincides with the thermal stress caused by the unevenness of the temperature distribution after the completion of the accelerated cooling and after the accelerated cooling and after the completion of the hot calibration, The distribution is measured, and the residual stress generated in the steel sheet after air cooling is calculated. Such a method of calculating the residual stress due to the thermal stress is disclosed, for example, in Japanese Patent Publication No. 4-8128. In the above method, a plate surface temperature profile measuring the plate surface temperature distribution of the steel sheet at the accelerated cooling facility or the hot calibration output side is provided as a temperature measuring means, and the temperature distribution of the plate surface temperature of the steel sheet is And the residual stress distribution occurring in the steel sheet after air cooling is calculated from the measurement results.

Then, based on the residual stress, for example, it is judged whether or not buckling occurs by comparison with the buckling critical stress calculated as described later. In addition, for the steel plate determined to have buckled, a heat treatment condition And the leveler processing conditions are compared with, for example, the buckling critical stress as described above, and the residual stress is reduced.

However, the conventional method can not prevent the occurrence of the defective shape. Therefore, in the present invention, the flatness of the surface of the steel sheet is measured immediately before or immediately after the measurement of the surface temperature distribution of the steel sheet, and the shape change of the steel sheet after air cooling is predicted according to the distribution of the residual stress and the measurement result of the flatness , The shape change of the steel sheet is predicted by correcting the amount of heat strain calculated from the distribution of the residual stress by the amount of strain after air-cooling calculated from the measurement result of the flatness. Specifically, the amount of strain after air cooling is calculated by the following method.

FIG. 1 is a schematic view showing a corrugated shape of a hot-rolled steel sheet.

(A) shows a sectional view, and (B) shows a perspective view.

The flatness defect of the hot-rolled steel sheet 10 is caused by a non-uniform distribution in the width direction of the longitudinal stretch. It is known that the difference DELTA epsilon _WAVE between elongation at the center of the plate width and at the end of the plate is expressed by the following equation 2 with respect to the wave shown in Fig. (For example, Japanese Association of Steel Industries Association, 3rd Edition, Steel Manual III (1), pages 53, 54) (Formula 2)

△ ε _WAVE = elongation difference between the center of the plate width and the end of the plate

?: strain amount (wave height)

p: pitch

(L ₁ ≤ x ≤ L ₂ ) and the temperature profile of the plate surface is T (x, y) (x: plate length direction, y: plate width direction) (Y) of the thermal strain in the width direction can be calculated by the following expression (3), and the elongation difference DELTA epsilon _TH at the center of the width of the plate and the end of the plate is calculated . Therefore, the thermal strain amount calculation means 5 may perform the calculation in accordance with this calculation procedure.

(Equation 3)

Δε _TH (y): the widthwise direction distribution of thermal strain

ΔT _AVE (y): average temperature difference in the panel width direction

E: Young's rate from measured temperature to room temperature

a: average thermal expansion coefficient from measurement temperature to room temperature

T _RT : room temperature

W: Woodwork

t: pipe thickness

The elongation difference (?? _TOTAL ) after air cooling can be obtained by superimposing?? _WAVE and?? _TH , i.e., by the following equation. Therefore, the strain amount calculating means 6 may calculate according to this calculation procedure.

△ _TOTAL = △ ε _WAVE + △ ε _TH

From this result, the strain amount? 'After air cooling can be obtained according to the following expression (4). Therefore, the out-of-plane deformation amount computing means 7 may perform computation according to this computation procedure.

(Equation 4)

δ '= Strain amount after air cooling

With the above calculation method, it is possible to estimate the flatness of the hot-rolled steel sheet 10 after air-cooling with high accuracy even if the annual rolled steel sheet 10 to be measured is hot.

In the present invention, the term "defective shape" refers to a defective buckling due to a change in a method of placing, a flatness defect after arbitrarily cut, or a transverse bending after cutting. In any case, And / or heat treatment is performed, a plurality of processing conditions are set in advance, the shape change after the processing is predicted from the residual stress when the cold-calibrating treatment and / or the heat treatment is performed according to each processing condition, Is recommended. When the buckling failure, the flatness defect, and the transverse bending are individually evaluated, it is predicted that the buckling failure will occur. In the case of performing the processing for preventing the buckling failure, Or transverse bending, it is necessary to select the processing condition considering the above-mentioned buckling defect, flatness defect and transverse bending in total.

As a manufacturing apparatus for realizing the above-described manufacturing method, a cold-rolled steel sheet manufacturing apparatus for cold-calibrating and / or heat-treating a steel sheet judged to have a defective shape in a steel sheet after air- A calculation means for calculating a shape variation amount of the steel sheet after air cooling in accordance with the flat surface temperature distribution by the flat surface temperature measurement means and the flatness degree by the flatness measurement means, Means for judging whether or not the steel sheet has passed the predetermined shape change amount by comparing the predicted shape change amount with a predetermined determination criterion; And a process condition selecting means for selecting a process condition for suppressing the shape change amount of the hot rolled steel sheet It is recommended to employ a manufacturing device.

FIG. 2 is a schematic explanatory view showing an apparatus configuration suitable for the practice of the present invention, and includes a sheet surface temperature measuring means 1 such as a scanning type temperature gauge for measuring a sheet surface temperature profile of a hot-rolled steel sheet 10, A flatness measuring means 2 for measuring the flatness of the hot-rolled steel sheet 10 by means of a laser distance meter (distance meter) on the downstream side of the measuring device 1, An operation processing control device 3 such as a computer, and a display device 4 such as a CRT for displaying information such as calculation results of the operation processing control device 3 on the screen.

Since the hot-rolled steel sheet 10 to be measured is a hot-rolled high-temperature steel sheet which is rolled out at a predetermined speed after being rolled in a hot-rolling line, the both measuring means 1 and 2 do not contact the plate surface temperature profile and the flatness It is preferable to measure it.

The arithmetic processing control device 3 is provided with a thermal strain amount calculating means 5 for calculating the amount of thermal strain generated after air cooling of the hot-rolled steel sheet 10 based on the sheet surface temperature profile measured by the sheet surface temperature measuring means 1, A strain amount calculating means 6 for calculating the strain amount of the hot-rolled steel sheet 10 in accordance with the flatness measured by the means 2, and a strain amount calculating means 6 for calculating a strain amount based on the thermal strain amount by the heat strain amount calculating means 5 and the strain amount by the strain amount calculating means 6 And an out-of-plane deformation amount calculating means 7 for estimating the out-of-plane deformation amount (out-of-plane deformation amount) of the hot-rolled steel sheet 10 after air cooling by correcting the flatness measured by the flatness measuring means 2.

3 is a schematic explanatory view showing a manufacturing line according to the present invention. For example, in the case of producing a hot-rolled steel sheet by using a slab, after being heated by a heating furnace, the scale formed on the surface is removed, and hot rolling is performed and accelerated cooling is performed. Hot calibration is carried out as required, and the temperature distribution on the surface of the steel sheet is measured according to a temperature profile meter or the like, and the flatness of the surface of the steel sheet is measured from the flatness meter (flatness meter). The obtained temperature data and shape data are sent to a computer, and the flatness after cooling is predicted according to the prediction method and the like, and whether or not buckling has occurred is determined according to a method described later. Those that satisfy the evaluation criteria of the shape are shipped as they are, and those that are not satisfactory are judged as unacceptable products and are shipped when the cold calibration and / or heat treatment is performed by a method described later and the shape evaluation criteria are satisfied.

[I] The judgment method [A] [B] which is preferable in judging whether or not a buckling wave (buckling wave) will occur when the method of mounting a steel sheet by a variable is varied will be described.

For example, when judging whether a buckling wave is generated or not from the residual stress distribution σ _act calculated by the method disclosed in Japanese Patent Publication No. Hei 4-8128, etc. when the method of depositing the steel sheet is varied, the residual stress It is possible to determine whether or not the buckling critical stress is exceeded. The buckling stress is a threshold varies depending on the distribution pattern of the residual stress σ _act (x, y) as described later. Therefore, a simple predictive equation (simple predictive equation) of the buckling critical stress corresponding to the distribution pattern of the residual stress σ _act (x, y) is constructed, and the buckling judgment is made by comparison with the calculation result with this predictive equation. The derivation process of this prediction equation is as follows.

First, as shown in FIG. 4, a coordinate system in which the plate length direction is x and the width direction is y is set for the steel plate having the plate length L and the plate width b. According to the buckling theory, ΔT and bending strain energy ΔV, which form σ _act (x, y) acting on the center plane in the plate thickness direction of the steel sheet when the warpage w occurs, , And (5).

(Equation 5)

E: Rate, v: Loss ratio, t: Plate thickness

Here, when the delta T is greater than the bending strain energy DELTA V to an arbitrary warping shape, the scallop deformation occurs, that is,

Buckling deformation occurs when? T /? V? 1.

In constructing the prediction equation of the buckling critical stress at the bottom of the above consideration, first, the area of the plate surface is divided by the average of the residual stress σ _act (x, y) into the compression region Ω ₂ and the other region Ω ₂ . Then, the residual stress distribution in the buckling threshold condition σ _cr (x, y) = η σ _act to be a relation between (x, y) established, and buckling the mean of σ _cr (x, y) in the region Ω ₁ The critical stress is σ _cr ^(-) . (6) is established at the buckling critical point, the following equation (7) is derived from the equations (4) and (5).

(Equation 6)

However, △ _act ^(-) = σ _act - σ _act ^(-)

? _Act ? ⁽⁺⁾ =? _Act -? _Act ⁽⁺⁾

σ _act ^(-) : average of σ _{act in} the region Ω ₁

σ _act ⁽⁺⁾ : average of σ _{act in} the region Ω ₂

η: Unknown number indicating the ratio of the residual stress of the steel plate to the buckling residual stress,

(Equation 7)

Σ _{0 is} an integer determined by the steel sheet size, buckling mode, and area dividing method, and the buckling critical stress when the residual stress distribution is approximated to a rectangular shape

(Discretization), and σ _cr ^(-)

(Equation 8)

However, F (x, y) =? (? W /? X) ² .

Also, the warp w is expressed as a polynomial about x, y,

(Equation 9)

Σ _cr ^(-) is given by

[Equation 10]

di, en: unspecified number determined by buckling mode ω

Δx and Δy are obtained by discretization intervals in the x and y directions, respectively. The uncertainty number in Eq. (10) can be determined for each buckling mode by comparison with the buckling analysis results obtained by FEM, etc. ^, and the relationship between σ _cr ^(-) and σ _act ⁽⁺

If σ _act ^(-) ≥ σ _cr ^(-) holds, it is judged that buckling has occurred.

The accuracy of the above-described buckling prediction equation is shown below.

Here,

However, a ₀ , a ₁ , a ₂ , b ₀ , b ₁ , and b ₂ are integers.

For a steel plate having various residual stress distributions σ _act (x, y), the buckling critical stresses were calculated from the above equation (10) and the FEM analysis, respectively. The results of this comparison are shown in FIG. It can be seen that the buckling critical stress can be predicted with high accuracy for a steel plate having an arbitrary stress distribution.

On the other hand, there has been known a method of calculating a buckling critical stress by considering a residual stress distribution as a shape compression of a flat plate which can be analyzed close to a sphere in a compression region. In order to verify the accuracy of the prediction method, the residual stress distribution pattern was varied and compared with the buckling critical stress obtained using FEM. The results are shown in FIG. As a result of the calculation in the FEM, the buckling critical stress σ _cr ^(-) largely changes according to the residual stress pattern in the compression region at the end of the plate, while the simplification method of the present invention in which the residual stress in the compression region is close to a sphere , The upper part of the distribution pattern is not reflected and becomes constant, and does not have sufficient precision for practical application. On the other hand, FEM analysis using a large computer is necessary for buckling judgment with high precision, and it is difficult to apply it to an open line.

Therefore, in this embodiment, with respect, for example as in the above (8) wherein a residual stress distribution in the buckling critical stress σ ₀ corresponding to when the closer to the sphere over the entire with the compression region Ω ₁ and the other region Ω ₂ Further, the residual stress (2) on the right side of Expression (8)] is added to the distribution of the residual stress distribution according to the distribution state, it is possible to calculate σ _cr ^(-) corresponding exactly to the actual residual stress distribution. Thus, the buckling critical stress can be predicted with high precision and also with a simple method.

In the above method, it is possible to determine buckling easily for any steel sheet having a residual stress distribution in the plate surface.

The steel plate judged as buckling occurrence is calibrated by the cold roller leveler. At this time, the residual stress σ _act ^af after calibration in the roller leveler,

However, λ _f: and can be calculated as a reduction coefficient of the residual stress in the leveler processing conditions Λ _f, of about σ _act ^af, as described above, the region where the residual stress after the correction of the printing plate mean that the compression Ω ₁ 'And the other region? ₂ ', and calculates the buckling critical stress? _Cr ^{af (-)} defined as above.

Then, for each leveler processing condition Λ f,

It is determined that buckling does not occur when the buckling is not established. Based on these determination results, the leveler processing condition in which buckling does not occur, that is,

, And by performing roller leveler calibration under the condition Λ f, it is possible to ship the steel plate as a steel plate which does not generate a buckling wave even if the mounting method varies in various ways.

In this case, the buckling critical stress σ _cr ^(-) / σ _cr ^{af (-)} may be obtained at the time of buckling judgment, and the determination may be made by comparison with them. The buckling occurrence may be determined based on the magnitude relationship between? T and? V.

It is indispensable to reduce the residual stress in the steel sheet within a range that does not exceed the buckling critical stress after the cutting method and the case where the method of stacking the steel sheet is varied. Particularly, in a steel sheet subjected to water cooling such as an accelerated cooling type steel sheet and a quenched type steel sheet, uneven residual stress is liable to occur in the steel sheet due to a temperature variation due to uneven cooling in the sheet surface during water cooling, It is extremely high.

As means for reducing the residual stress in the steel sheet, such a method may be employed in which a heat treatment is performed and a roller leveler correction in cold is adopted.

[II] Calibration method [C], which is preferably employed in the case of calibrating by the roller leveler in the cold, will be described.

It is known that in the case of calibrating by the roller leveler in the cold, the residual stress can be reduced in accordance with this calibration condition. Therefore, it is conceivable that the shape-unstable steel sheet as described above is calibrated by the roller leveler in the cold, thereby reducing the residual stress and shipping the steel sheet. However, since it is not known whether the degree of the residual stress reduction due to the correction at this time becomes smaller than the buckling critical stress and becomes a state in which buckling does not occur, proper correction processing conditions can not be set. As a result, it is difficult to identify an unstable steel sheet during manufacture of the steel sheet, thereby preventing occurrence of buckling.

However, the conventional roller leveler correction is usually applied only when the shape defect remains after the hot correction, and also when the steel plate in which the defective shape remains is calibrated, the setting of the calibration condition is determined considering only the steel plate- The residual stress was not considered. For this reason, depending on the state of residual stress in the steel sheet, a problem due to the buckling deformation as described above may occur.

The correction method [C] that solves such a problem is based on theoretical analysis based on the mechanics under various conditions for changes in the shape of the steel sheet and the residual stress when the steel sheet having the residual stress is calibrated to the cold leveler It is based on the insights gained as a result of detailed investigation.

First, the calculation of the residual stress distribution after calibration will be described in detail. FIG. 7 is an example of an analysis of a change in residual stress due to calibration. (a) is the residual stress after calibration, and (b) before calibration. Figure 8 also shows the results of the analysis of the various residual stress distributions in terms of the residual stresses before and after calibration at each location in the plate. From these results, it can be seen that the residual stress at each position in the plate surface before and after calibration has a slight deviation, but is located on the same curve without depending on the pattern of the residual stress distribution before calibration under the same calibration conditions. Therefore, the residual stress σ _a after calibration and the residual stress σ _b after calibration

. ≪ / RTI > The equation (11) is approximate,

However, C ₀ is an integer, and? Is a reduction coefficient of the residual stress.

In addition, the integral term C _O is approximated as C ₀ = 0 considering that the relationship between σ _a and σ _b is approximately the origin through the curve G as shown in FIG. 8, and the residual stress after calibration The distribution σ _a (x, y) is obtained by using the reduction coefficient η of the residual stress,

As shown in FIG. In addition, the reduction coefficient? Of the residual stress has an approximate linear relationship with respect to the residual stress? _B before the correction as in Fig. 9,

. Therefore, as shown in FIG. 9, the coefficients a and b can be calculated by using the equation (12) previously obtained by mechanical theoretical analysis or the like according to the material characteristics such as the reduction pattern, the steel sheet size, The residual stress distribution can be obtained. The residual stress distribution before calibration used at this time can be estimated from the plate surface temperature profile of the steel sheet in the hot state immediately after passing through the hot leveler, for example, using the formula (8) described in Japanese Patent Publication No. 4-8128.

Then, by performing the buckling judgment with respect to the residual stress distribution after the correction determined as described above, it is possible to obtain a reduction pattern capable of preventing the occurrence of the shape defect due to the buckling deformation.

That is, as described above, the residual stress distribution after calibration is obtained to determine buckling for all of a plurality of predetermined downward pressing down patterns r _i consisting of the input side, the output reduction amount, and the bending amount of the correcting roll, If the pressing down pattern r _OPT which is within the range of the minimum pressing down pattern r _min and the maximum pressing down pattern r _max is calibrated from among them, as will be described below, The occurrence of defective shape due to buckling deformation can be prevented.

In this case, the buckling determination can be performed by theoretical analysis by the finite element method and the method disclosed in Japanese Patent Application Laid-Open No. 8-187505.

[III] Lastly, the above-mentioned heat treatment method [D] will be described.

Since the shape change of the steel sheet after cutting is a phenomenon due to the residual stress, if the residual stress state of the steel sheet is known, it is possible to estimate a shape change such as a transverse bending at the time of cutting. (X, y) (x: position in the longitudinal direction of the plate, y: longitudinal direction of the steel plate) is calculated by the method shown in Japanese Patent Application Laid-Open No. 4-8128 and Japanese Patent Publication No. 4-8129, The deformation amount? ₁ after the cutting can be obtained from the balance of the stress and the moment in the member after the cutting based on the position in the width direction).

Therefore, if the residual stress after the heat treatment can be calculated according to the heat treatment condition (temperature, time), it is possible to determine the heat treatment condition capable of reducing the deformation amount? At the time of cutting to the allowable amount? ₀ , and heat treatment is performed under this heat treatment condition The deformation amount? ₁ after cutting can be controlled within the allowable amount? ₀ .

Next, a method of estimating the residual stress before and after the heat treatment will be described in detail.

(1) Estimation of residual stress before heat treatment

The residual stress before heat treatment The residual stress immediately after air cooling to room temperature after quenching or accelerated cooling and the residual stress change due to the calibration are superimposed when cold calibrating is performed.

(1-i) Method for Estimating Residual Stress of Steel Sheet Immediately After Cooling to Room Temperature After Accelerated Cooling

[A] In the case of a steel sheet subjected to accelerated cooling

Inside the accelerated cooling steel sheet, the stress caused by the accelerated cooling process, the stress due to the subsequent hot calibration, and the temperature distribution in the plate surface formed during the accelerated cooling process, the thermal stress generated during cooling to room temperature by air cooling Stress is formed. However, as shown in FIG. 10, or as shown in the literature ("Kobbe Seiki Kogebo", Vol. 35, No. 4, page 87), Japanese Patent Publication No. 4-8128 and Japanese Patent Publication No. 4-8129, (The average value in the thickness direction) of the steel sheet estimated from the temperature distribution of the steel plate coincides with the residual stress distribution estimated from the temperature distribution in the plate surface after the accelerator cooling or after the accelerated cooling, ? _c (x, y) can be estimated from the in-plane temperature distribution T (x, y) after the hot leveler after the accelerated cooling stop or the accelerated cooling. However, as described above, it is necessary to correct the residual stress? _C (x, y) with information obtained by a flatness meter (flatness meter).

[B] In case of steel plate

In the case of quenching, unlike accelerated cooling, the steel sheet is extracted in a uniformly heated state by a heating furnace, and then water cooling is performed immediately. Therefore, if the steel sheet size is determined, The temperature history at each position in the plate surface after quenching is approximately the same, The residual stress after quenching is also approximately the same. For this reason, it can be obtained in advance, for example, by thermo-elasto-plastic analysis using the finite element method as shown in FIG. 11. Of course, as in the case of accelerated cooling steel sheets, it is possible to estimate on the basis of the temperature distribution on the surface of the steel sheet. In other words, in the temperature distribution described in the Patent Publication Hei No. 4-8128, Unexamined Patent Publication No. Hei 4-8129 T (x, y) a (temperature distribution in the subsequent transformation finished) the temperature distribution after the quenching, and the transformation strain ε _ph Can be obtained by overlapping. For example, it can be obtained by the following formula.

α: average coefficient of thermal expansion

T _RT : room temperature

E: Rate

W: Panel width

t: plate thickness

However, it is necessary to correct the residual stress? _C (X, Y) with the information obtained by the flatness meter.

(1-ii) Change in Residual Stress due to Cold Calibration

When cold-calibrating is performed before the heat treatment, it largely changes from the residual stress state after water-cooling as described above, and becomes a different distribution. Taking this as an example of press calibration, FIG. 9 shows a stress state when an accelerated cooling steel plate having a yield stress of 50 kgf / mm 2 and a plate thickness of 55 mm is calibrated (three-point bending) at a supporting point distance of 500 mm. In Fig. 12, σ _av represents the average residual stress before calibration.

It can be seen that the residual stress after the press calibration largely changes due to the press-in amount (push-in amount) and the stress state before the calibration. Therefore, it can be said that it is indispensable to apply cold resisting in addition to the residual stress after air cooling.

Therefore, the method of calculating the stress change at the time of cold calibration will be described.

The stress distribution in the thickness direction when air-cooled to room temperature after water cooling is dependent on the cooling rate in the water-cooling process. For example, when water is cooled at a cooling rate of 9 deg. C / sec (cooling start temperature: 900 deg. : 450 占 폚, cooling time: 50 seconds), σ _g is _obtained as shown in FIG. Therefore, the stress σ _a0 generated in the steel sheet

. The stress state σ _p after calibrating the steel sheet whose initial stress is the stress is calculated by the initial stress σ _a0 of the steel sheet and the calibration condition Λ _P (x, y) at the position to be corrected

σ _P = G (σ _a0 (x, y, z), Λ _P (x, y)).

The plastic strain ε _P (x, y) generated by the three-point bending in the press calibration can be calculated from the residual stress σ _C (x, y) after air cooling and the water pressure of the press calibration condition Λ _P (x, y) Lt; / RTI >

(x, y) and Λ p (X, y).

Therefore, the residual stress σ p after press calibration is

As shown in FIG.

(2) Change of residual stress in heat treatment process

In the present invention, the change of the residual stress in the heat treatment process is determined by the surface properties of the substrate before the heat treatment and the creep characteristics during the heat treatment.

(1) Influence of surface properties due to maintenance before heat treatment

When the surface is trimmed, the thermal stress is generated because the temperature rise characteristics are different between the cleaned portion and the untreated portion, which greatly affects the residual stress after the heat treatment. FIG. 14 is a result of measuring the temperature history of the steel sheet, and it can be seen that the temperature history is different between the cleaned area and the untreated area. This is because the thermal conductivity on the surface of the steel sheet changes due to maintenance. In the case of FIG. 14, it can be understood that the portion of the cleaning is about 30% smaller. According to this influence, the residual stress after the heat treatment does not cause a large stress deviation in the case of a non-treated steel sheet, but in the case of a treated steel sheet, a stress deviation of about 10 kgf / mm < 2 & . From this, it can be understood that accurate residual stress can not be evaluated unless the influence is calculated as in the present invention.

② Effect of creep deformation during heat treatment

When a steel sheet having a residual stress is left at a high temperature, a permanent stress called a creep is generated and the residual stress inside the steel sheet is reduced. The creep strain ε _C generated at this time is, at a temperature of 550 ° C. or higher

A, n: integer

Lt; / RTI > The residual stress after the heat treatment under ideal conditions is determined by the generation behavior of this creep strain.

The residual stress behavior in the heat treatment process is determined by the following (1) and (2).

(i) From the initiation of heat treatment to reaching the holding temperature, the thermal stress caused by different surface properties on the surface of the steel sheet overlaps with the residual stress before heat treatment, and at the same time, stress relaxation occurs simultaneously with creep deformation at high temperatures.

(ii) As a result, the stress state immediately before the steel sheet temperature becomes the holding temperature is determined, and creep deformation occurs according to the heat treatment holding temperature and the holding time, and the residual stress of the steel sheet after the heat treatment is reduced.

Therefore, a method of calculating the residual stress after the heat treatment after the above-mentioned process will be described in detail.

The residual stress σ ₁ after heat treatment can be estimated by performing hot-point carbon-carbon segregation, but calculation time increases and becomes longer. Next, a method of simply calculating the residual stress after the heat treatment with high accuracy will be described.

The residual stress σ ₁ after the heat treatment is determined by the residual stress (initial stress) σ ₀ before heat treatment and the heat treatment conditions (heat treatment temperature and holding time). After dull hang initial stress σ ₀₀ on a steel sheet under the state in which the constraint that both ends of the steel sheet in the heat treatment temperature T ₀ t minute (分) stress σ ₁₁ after being seen for is from 13 expression

E ₁ : Rate at holding temperature

E ₀ : Rate at room temperature

. σ ₁₁ is the residual stress after heat treatment at each position. Considering the conditions of force and moment in the entire steel plate, residual stress σ ₁ after heat treatment appears as follows.

In the calculation of σ ₀₀ , the influence of the polishing is considered as the residual stress σ _B generated by the heat treatment as the residual stress before the heat treatment. Specifically, σ ₀₀ is the value obtained by adding the residual stress before heat treatment to the residual stress generated by the heat treatment (σ _k + σ _P ), and (σ _k + σ _p ) exceeds the yield stress σ _y ^RT at room temperature However,

σ ₀₀ = MIN (σ _H + σ _P , σ _y ^RT ), and σ ₀₀ can be set.

The residual stress σ ₁ of the steel sheet after heat treatment is the residual stress σ ₀ before heat treatment determined according to the residual stress σ _c and the calibration condition Λ formed after quenching or accelerated cooling,

And heat treatment stress, mainly flaw ( ) Of the surface of steel sheet due to maintenance And stress caused by creep deformation are superimposed on each other

T _heat : Heat treatment temperature

t _hold : heat treatment time

A, n: Creep constant

And the heat treatment condition capable of controlling the amount of deformation? Of the cutting or the like within the allowable range? ₀ can be calculated by using the above-described various expressions.

Next, the present invention will be described in more detail by way of examples. The following embodiments are not intended to limit the present invention, and design changes according to the purposes of the preceding and following items are included in the technical scope of the present invention.

[Example]

Example 1

In the following Table 1, the flatness in temperature and the temperature profile were measured by using two types of steel sheets A and B showing the sizes, and the flatness after cooling was predicted.

FIG. 16 shows the temperature distribution in the width direction of the steel sheets immediately before the flatness measurement of the steel sheets A and B, and the temperature deviation of the steel sheet A shown in FIG.

The temperature deviation of the steel plate B shown in (B) was 17.1 占 폚. In the method of the present invention, the thermal strain amount is calculated from the distribution of the residual stress of the steel sheet generated after air cooling, based on the above-mentioned temperature distribution in the direction of the widthwise direction, and the measurement result of the flatness in the warm state is corrected , And the flatness after cooling was predicted. The results are shown in Table 2.

According to the conventional method, the flatness measurement result in the warm state is used as it is as flattened after cooling, and the wave height (delta in Fig. 1) in the shape after hot rolling and air cooling as shown in Table 2 is (Medium wave: medium wave) in steel plate A and 31.5 mm (short wave: end wave) in steel plate B, and there is a large difference between the result of measurement of flatness in warmth (value in the column of conventional method).

On the other hand, according to the method of the present invention, the medium wave of the steel plate A having a wave height of 9.3 mm as measured after air cooling was approximately 9.5 mm. Further, with respect to shortwave of the steel plate B having a wave height of 31.5 mm as measured after air cooling, it was predicted to be 31.8 mm with an error of only about 1%.

According to the method of the present invention which considers shape blurring due to the temperature deviation, it can be understood that the flatness after air cooling can be estimated with high accuracy.

[Example 2]

A hot-rolled steel sheet is manufactured by cold-calibrating and heat-treating a 50-kilo grade steel sheet of TMCP (Thermo Mechanical Control Process) type having a plate thickness of 8 to 32 mm, a plate width of 2500 to 4200 mm and a plate length of 10 to 24 m Method), buckling, flatness and transverse bending after cutting were investigated. Next, using the same steel plate, a plate surface temperature profile meter and a flatness meter were installed after the hot calibration, and the buckling and transverse bending were predicted by the data of the plate surface temperature profile system, and the flatness data And the occurrence of defective shape was predicted by cold calibrating and / or heat treatment (inventive method 1) to investigate the occurrence rate of the defective shape. In the same manner as in the method 1 of the present invention, except that the same steel sheet was used to select a condition in which a defective shape was predicted, (Method 2 of the present invention), and the occurrence rate of the defective shape was examined. The results are shown in Table 3. (The number of steel sheets n to be inspected is 100)

(Table 3)

It can be seen that the occurrence of defects in the conventional method is seen, but the occurrence rate of the defective shape can be greatly reduced in the present invention method.

Since the present invention is constituted as described above, it is possible to precisely predict the buckling deformation and the shape change after the cutting process due to the difference in the flatness of the hot-rolled steel sheet and the mounting method, It is possible to provide a manufacturing method and a manufacturing and disposing method capable of preventing the occurrence of shape defects.

Claims (5)

The surface temperature distribution (plate surface temperature distribution) of the steel sheet is measured prior to air-cooling the hot-rolled steel sheet subjected to the accelerated cooling process or the hot calibration process, thereby calculating the residual stress of the steel sheet to be generated after air cooling, The cold rolled steel sheet which has been judged to be deformed due to the shape change of the steel sheet from the distribution condition and the process condition of the steel sheet is subjected to cold calibrating treatment and / or heat treatment to remove the defective shape in advance; Further, the flatness of the surface of the steel sheet is measured before and after the measurement of the temperature distribution of the surface of the steel sheet; Further, by correcting the amount of heat strain (heat strain amount) calculated from the calculated distribution of the residual stress to the amount of strain after air-cooling calculated from the measurement result of the flatness, And removing the hot rolled steel sheet. The method of manufacturing a hot-rolled steel sheet according to claim 1, wherein the defective shape removing method is a defective occurrence removing method such as buckling, defective flatness or lateral bending after cutting. The method of manufacturing a steel plate according to claim 2, wherein, in performing the cold-calibrating process and / or the heat-treating process on the steel plate determined to have any of the shape defects, a plurality of process conditions are set in advance, (Manufacturing method) of a hot-rolled steel sheet in which a shape change after a treatment is predicted from residual stress when a heat treatment is performed and / or a heat treatment is carried out, and as a result, a processing condition which does not cause any shape defect is selected. The steel sheet according to any one of claims 1 to 3, wherein the shape of the steel sheet when the flatness is measured is a steel sheet having a pitch p of height 隆 at a pitch p and a difference in elongation at the center of the sheet- Is DELTA epsilon _WAVE , and the width direction distribution of thermal stress is DELTA epsilon _TH , The amount of strain after air cooling:? 'Is calculated by the following formula. The temperature distribution of the surface of the steel sheet is measured prior to the air cooling of the hot rolled steel sheet subjected to the accelerated cooling process or the hot calibration process and the residual stress of the steel sheet to be generated after air cooling is calculated, The apparatus for manufacturing a hot-rolled steel sheet according to any one of the preceding claims, wherein a shape change of the steel sheet is predicted from a treatment condition, A plate surface temperature measuring means (plate surface temperature measuring means) for measuring a plate surface temperature distribution of the steel sheet; Flatness measuring means (flatness measuring means) for measuring the flatness of the surface of the steel sheet; Calculating means (arithmetic means) for predicting the shape change amount of the steel sheet after air-cooling by means of the sheet surface temperature distribution data obtained by the sheet surface temperature measuring means and the flatness data obtained by the flatness measuring means; Acceptance determination means (acceptance determination means) for comparing the predicted shape change amount with a predetermined determination criterion to determine acceptance; (Processing selection means) for selecting a processing condition for suppressing the shape change amount of the steel sheet within the acceptance criterion from a plurality of preset processing conditions for the cold correction processing and / or the heat treatment, And the occurrence of a defective shape is removed in advance from the prediction. (Manufacturing Apparatus) for Manufacturing a Hot Rolled Steel Sheet.